diff --git a/lib/systems/default.nix b/lib/systems/default.nix
index 00f7f3523aa4..ec0106a13dac 100644
--- a/lib/systems/default.nix
+++ b/lib/systems/default.nix
@@ -102,6 +102,7 @@ let
         else if final.isMusl                  then "musl"
         else if final.isUClibc                then "uclibc"
         else if final.isAndroid               then "bionic"
+        else if final.isLLVMLibc              then "llvm"
         else if final.isLinux  /* default */  then "glibc"
         else if final.isFreeBSD               then "fblibc"
         else if final.isOpenBSD               then "oblibc"
diff --git a/lib/systems/inspect.nix b/lib/systems/inspect.nix
index ebc7ab366876..de4df13cd811 100644
--- a/lib/systems/inspect.nix
+++ b/lib/systems/inspect.nix
@@ -113,6 +113,7 @@ rec {
     isGnu          = with abis; map (a: { abi = a; }) [ gnuabi64 gnuabin32 gnu gnueabi gnueabihf gnuabielfv1 gnuabielfv2 ];
     isMusl         = with abis; map (a: { abi = a; }) [ musl musleabi musleabihf muslabin32 muslabi64 ];
     isUClibc       = with abis; map (a: { abi = a; }) [ uclibc uclibceabi uclibceabihf ];
+    isLLVMLibc     = [ { abi = abis.llvm; } ];
 
     isEfi = [
       { cpu = { family = "arm"; version = "6"; }; }
diff --git a/lib/systems/parse.nix b/lib/systems/parse.nix
index fa3cc02647ae..e3e6ef16c6b0 100644
--- a/lib/systems/parse.nix
+++ b/lib/systems/parse.nix
@@ -416,6 +416,9 @@ rec {
     uclibceabihf = { float = "hard"; };
     uclibc       = {};
 
+    # LLVM libc
+    llvm         = {};
+
     unknown = {};
   };
 
diff --git a/pkgs/development/compilers/llvm/common/default.nix b/pkgs/development/compilers/llvm/common/default.nix
index c9886052d632..27e7e05e4d1c 100644
--- a/pkgs/development/compilers/llvm/common/default.nix
+++ b/pkgs/development/compilers/llvm/common/default.nix
@@ -1075,135 +1075,148 @@ let
           relative = "compiler-rt";
         });
     in
-    {
-      compiler-rt-libc = callPackage ./compiler-rt (
-        let
-          # temp rename to avoid infinite recursion
+    (
+      {
+        compiler-rt-libc = callPackage ./compiler-rt (
+          let
+            # temp rename to avoid infinite recursion
+            stdenv =
+              # Darwin needs to use a bootstrap stdenv to avoid an infinite recursion when cross-compiling.
+              if args.stdenv.hostPlatform.isDarwin then
+                overrideCC darwin.bootstrapStdenv buildLlvmTools.clangWithLibcAndBasicRtAndLibcxx
+              else if args.stdenv.hostPlatform.useLLVM or false then
+                overrideCC args.stdenv buildLlvmTools.clangWithLibcAndBasicRtAndLibcxx
+              else
+                args.stdenv;
+          in
+          {
+            patches = compiler-rtPatches;
+            inherit stdenv;
+          }
+          // lib.optionalAttrs (stdenv.hostPlatform.useLLVM or false) {
+            libxcrypt = (libxcrypt.override { inherit stdenv; }).overrideAttrs (old: {
+              configureFlags = old.configureFlags ++ [ "--disable-symvers" ];
+            });
+          }
+        );
+
+        compiler-rt-no-libc = callPackage ./compiler-rt {
+          patches = compiler-rtPatches;
+          doFakeLibgcc = stdenv.hostPlatform.useLLVM or false;
           stdenv =
             # Darwin needs to use a bootstrap stdenv to avoid an infinite recursion when cross-compiling.
-            if args.stdenv.hostPlatform.isDarwin then
-              overrideCC darwin.bootstrapStdenv buildLlvmTools.clangWithLibcAndBasicRtAndLibcxx
-            else if args.stdenv.hostPlatform.useLLVM or false then
-              overrideCC args.stdenv buildLlvmTools.clangWithLibcAndBasicRtAndLibcxx
+            if stdenv.hostPlatform.isDarwin then
+              overrideCC darwin.bootstrapStdenv buildLlvmTools.clangNoLibcNoRt
             else
-              args.stdenv;
-        in
-        {
-          patches = compiler-rtPatches;
-          inherit stdenv;
-        }
-        // lib.optionalAttrs (stdenv.hostPlatform.useLLVM or false) {
-          libxcrypt = (libxcrypt.override { inherit stdenv; }).overrideAttrs (old: {
-            configureFlags = old.configureFlags ++ [ "--disable-symvers" ];
-          });
-        }
-      );
+              overrideCC stdenv buildLlvmTools.clangNoLibcNoRt;
+        };
 
-      compiler-rt-no-libc = callPackage ./compiler-rt {
-        patches = compiler-rtPatches;
-        doFakeLibgcc = stdenv.hostPlatform.useLLVM or false;
-        stdenv =
-          # Darwin needs to use a bootstrap stdenv to avoid an infinite recursion when cross-compiling.
-          if stdenv.hostPlatform.isDarwin then
-            overrideCC darwin.bootstrapStdenv buildLlvmTools.clangNoLibcNoRt
+        compiler-rt =
+          if
+            stdenv.hostPlatform.libc == null
+            # Building the with-libc compiler-rt and WASM doesn't yet work,
+            # because wasilibc doesn't provide some expected things. See
+            # compiler-rt's file for further details.
+            || stdenv.hostPlatform.isWasm
+            # Failing `#include <term.h>` in
+            # `lib/sanitizer_common/sanitizer_platform_limits_freebsd.cpp`
+            # sanitizers, not sure where to get it.
+            || stdenv.hostPlatform.isFreeBSD
+          then
+            libraries.compiler-rt-no-libc
           else
-            overrideCC stdenv buildLlvmTools.clangNoLibcNoRt;
-      };
+            libraries.compiler-rt-libc;
 
-      compiler-rt =
-        if
-          stdenv.hostPlatform.libc == null
-          # Building the with-libc compiler-rt and WASM doesn't yet work,
-          # because wasilibc doesn't provide some expected things. See
-          # compiler-rt's file for further details.
-          || stdenv.hostPlatform.isWasm
-          # Failing `#include <term.h>` in
-          # `lib/sanitizer_common/sanitizer_platform_limits_freebsd.cpp`
-          # sanitizers, not sure where to get it.
-          || stdenv.hostPlatform.isFreeBSD
-        then
-          libraries.compiler-rt-no-libc
-        else
-          libraries.compiler-rt-libc;
+        stdenv = overrideCC stdenv buildLlvmTools.clang;
 
-      stdenv = overrideCC stdenv buildLlvmTools.clang;
+        libcxxStdenv = overrideCC stdenv buildLlvmTools.libcxxClang;
 
-      libcxxStdenv = overrideCC stdenv buildLlvmTools.libcxxClang;
-
-      libcxx = callPackage ./libcxx (
-        {
-          patches = lib.optionals (lib.versionOlder metadata.release_version "16") (
-            lib.optional (lib.versions.major metadata.release_version == "15")
-              # See:
-              #   - https://reviews.llvm.org/D133566
-              #   - https://github.com/NixOS/nixpkgs/issues/214524#issuecomment-1429146432
-              # !!! Drop in LLVM 16+
-              (
-                fetchpatch {
-                  url = "https://github.com/llvm/llvm-project/commit/57c7bb3ec89565c68f858d316504668f9d214d59.patch";
-                  hash = "sha256-B07vHmSjy5BhhkGSj3e1E0XmMv5/9+mvC/k70Z29VwY=";
-                }
-              )
-            ++ [
-              (substitute {
-                src = ./libcxxabi/wasm.patch;
+        libcxx = callPackage ./libcxx (
+          {
+            patches = lib.optionals (lib.versionOlder metadata.release_version "16") (
+              lib.optional (lib.versions.major metadata.release_version == "15")
+                # See:
+                #   - https://reviews.llvm.org/D133566
+                #   - https://github.com/NixOS/nixpkgs/issues/214524#issuecomment-1429146432
+                # !!! Drop in LLVM 16+
+                (
+                  fetchpatch {
+                    url = "https://github.com/llvm/llvm-project/commit/57c7bb3ec89565c68f858d316504668f9d214d59.patch";
+                    hash = "sha256-B07vHmSjy5BhhkGSj3e1E0XmMv5/9+mvC/k70Z29VwY=";
+                  }
+                )
+              ++ [
+                (substitute {
+                  src = ./libcxxabi/wasm.patch;
+                  substitutions = [
+                    "--replace-fail"
+                    "/cmake/"
+                    "/llvm/cmake/"
+                  ];
+                })
+              ]
+              ++ lib.optional stdenv.hostPlatform.isMusl (substitute {
+                src = ./libcxx/libcxx-0001-musl-hacks.patch;
                 substitutions = [
                   "--replace-fail"
-                  "/cmake/"
-                  "/llvm/cmake/"
+                  "/include/"
+                  "/libcxx/include/"
                 ];
               })
-            ]
-            ++ lib.optional stdenv.hostPlatform.isMusl (substitute {
-              src = ./libcxx/libcxx-0001-musl-hacks.patch;
-              substitutions = [
-                "--replace-fail"
-                "/include/"
-                "/libcxx/include/"
-              ];
-            })
-          );
-          stdenv =
-            if stdenv.hostPlatform.isDarwin then
-              overrideCC darwin.bootstrapStdenv buildLlvmTools.clangWithLibcAndBasicRt
-            else
-              overrideCC stdenv buildLlvmTools.clangWithLibcAndBasicRt;
-        }
-        // lib.optionalAttrs (lib.versionOlder metadata.release_version "14") {
-          # TODO: remove this, causes LLVM 13 packages rebuild.
-          inherit (metadata) monorepoSrc; # Preserve bug during #307211 refactor.
-        }
-      );
-
-      libunwind = callPackage ./libunwind {
-        patches = lib.optional (lib.versionOlder metadata.release_version "17") (
-          metadata.getVersionFile "libunwind/gnu-install-dirs.patch"
+            );
+            stdenv =
+              if stdenv.hostPlatform.isDarwin then
+                overrideCC darwin.bootstrapStdenv buildLlvmTools.clangWithLibcAndBasicRt
+              else
+                overrideCC stdenv buildLlvmTools.clangWithLibcAndBasicRt;
+          }
+          // lib.optionalAttrs (lib.versionOlder metadata.release_version "14") {
+            # TODO: remove this, causes LLVM 13 packages rebuild.
+            inherit (metadata) monorepoSrc; # Preserve bug during #307211 refactor.
+          }
         );
-        stdenv = overrideCC stdenv buildLlvmTools.clangWithLibcAndBasicRt;
-      };
 
-      openmp = callPackage ./openmp {
-        patches =
-          lib.optional (
-            lib.versionAtLeast metadata.release_version "15" && lib.versionOlder metadata.release_version "19"
-          ) (metadata.getVersionFile "openmp/fix-find-tool.patch")
-          ++ lib.optional (
-            lib.versionAtLeast metadata.release_version "14" && lib.versionOlder metadata.release_version "18"
-          ) (metadata.getVersionFile "openmp/gnu-install-dirs.patch")
-          ++ lib.optional (lib.versionAtLeast metadata.release_version "14") (
-            metadata.getVersionFile "openmp/run-lit-directly.patch"
-          )
-          ++
-            lib.optional (lib.versionOlder metadata.release_version "14")
-              # Fix cross.
-              (
-                fetchpatch {
-                  url = "https://github.com/llvm/llvm-project/commit/5e2358c781b85a18d1463fd924d2741d4ae5e42e.patch";
-                  hash = "sha256-UxIlAifXnexF/MaraPW0Ut6q+sf3e7y1fMdEv1q103A=";
-                }
-              );
-      };
-    }
+        libunwind = callPackage ./libunwind {
+          patches = lib.optional (lib.versionOlder metadata.release_version "17") (
+            metadata.getVersionFile "libunwind/gnu-install-dirs.patch"
+          );
+          stdenv = overrideCC stdenv buildLlvmTools.clangWithLibcAndBasicRt;
+        };
+
+        openmp = callPackage ./openmp {
+          patches =
+            lib.optional (
+              lib.versionAtLeast metadata.release_version "15" && lib.versionOlder metadata.release_version "19"
+            ) (metadata.getVersionFile "openmp/fix-find-tool.patch")
+            ++ lib.optional (
+              lib.versionAtLeast metadata.release_version "14" && lib.versionOlder metadata.release_version "18"
+            ) (metadata.getVersionFile "openmp/gnu-install-dirs.patch")
+            ++ lib.optional (lib.versionAtLeast metadata.release_version "14") (
+              metadata.getVersionFile "openmp/run-lit-directly.patch"
+            )
+            ++
+              lib.optional (lib.versionOlder metadata.release_version "14")
+                # Fix cross.
+                (
+                  fetchpatch {
+                    url = "https://github.com/llvm/llvm-project/commit/5e2358c781b85a18d1463fd924d2741d4ae5e42e.patch";
+                    hash = "sha256-UxIlAifXnexF/MaraPW0Ut6q+sf3e7y1fMdEv1q103A=";
+                  }
+                );
+        };
+      }
+      // lib.optionalAttrs (lib.versionAtLeast metadata.release_version "20") {
+        libc-overlay = callPackage ./libc {
+          isFullBuild = false;
+        };
+
+        libc-full = callPackage ./libc {
+          isFullBuild = true;
+        };
+
+        libc = if stdenv.targetPlatform.libc == "llvm" then libraries.libc-full else libraries.libc-overlay;
+      }
+    )
   );
 
   noExtend = extensible: lib.attrsets.removeAttrs extensible [ "extend" ];
diff --git a/pkgs/development/compilers/llvm/common/libc/default.nix b/pkgs/development/compilers/llvm/common/libc/default.nix
new file mode 100644
index 000000000000..eecae81bb433
--- /dev/null
+++ b/pkgs/development/compilers/llvm/common/libc/default.nix
@@ -0,0 +1,95 @@
+{
+  lib,
+  stdenv,
+  llvm_meta,
+  src ? null,
+  monorepoSrc ? null,
+  version,
+  release_version,
+  runCommand,
+  python3,
+  python3Packages,
+  patches ? [ ],
+  cmake,
+  ninja,
+  isFullBuild ? true,
+  linuxHeaders,
+}:
+let
+  pname = "libc";
+
+  src' = runCommand "${pname}-src-${version}" { } (''
+    mkdir -p "$out"
+    cp -r ${monorepoSrc}/cmake "$out"
+    cp -r ${monorepoSrc}/runtimes "$out"
+    cp -r ${monorepoSrc}/llvm "$out"
+    cp -r ${monorepoSrc}/${pname} "$out"
+  '');
+
+  stdenv' =
+    if stdenv.cc.isClang then
+      stdenv.override {
+        cc = stdenv.cc.override {
+          nixSupport = stdenv.cc.nixSupport // {
+            cc-cflags = lib.remove "-lunwind" stdenv.cc.nixSupport.cc-cflags;
+          };
+        };
+      }
+    else
+      stdenv;
+in
+stdenv'.mkDerivation (finalAttrs: {
+  inherit pname version patches;
+
+  src = src';
+
+  sourceRoot = "${finalAttrs.src.name}/runtimes";
+
+  nativeBuildInputs =
+    [
+      cmake
+      python3
+    ]
+    ++ (lib.optional (lib.versionAtLeast release_version "15") ninja)
+    ++ (lib.optional isFullBuild python3Packages.pyyaml);
+
+  buildInputs = lib.optional isFullBuild linuxHeaders;
+
+  outputs = [ "out" ] ++ (lib.optional isFullBuild "dev");
+
+  postUnpack = lib.optionalString isFullBuild ''
+    patchShebangs $sourceRoot/../$pname/hdrgen/yaml_to_classes.py
+    chmod +x $sourceRoot/../$pname/hdrgen/yaml_to_classes.py
+  '';
+
+  prePatch = ''
+    cd ../${finalAttrs.pname}
+    chmod -R u+w ../
+  '';
+
+  postPatch = ''
+    cd ../runtimes
+  '';
+
+  postInstall = lib.optionalString (!isFullBuild) ''
+    substituteAll ${./libc-shim.so} $out/lib/libc.so
+  '';
+
+  libc = if (!isFullBuild) then stdenv.cc.libc else null;
+
+  cmakeFlags = [
+    (lib.cmakeBool "LLVM_LIBC_FULL_BUILD" isFullBuild)
+    (lib.cmakeFeature "LLVM_ENABLE_RUNTIMES" "libc")
+  ];
+
+  # For the update script:
+  passthru = {
+    monorepoSrc = monorepoSrc;
+    inherit isFullBuild;
+  };
+
+  meta = llvm_meta // {
+    homepage = "https://libc.llvm.org/";
+    description = "Standard C library for LLVM";
+  };
+})
diff --git a/pkgs/development/compilers/llvm/common/libc/libc-shim.so b/pkgs/development/compilers/llvm/common/libc/libc-shim.so
new file mode 100644
index 000000000000..f3beea87680d
--- /dev/null
+++ b/pkgs/development/compilers/llvm/common/libc/libc-shim.so
@@ -0,0 +1 @@
+GROUP (@libc@ @out@/lib/libllvmlibc.a)
diff --git a/pkgs/development/compilers/llvm/default.nix b/pkgs/development/compilers/llvm/default.nix
index 569c0e2dd0d0..ee60db1e08a6 100644
--- a/pkgs/development/compilers/llvm/default.nix
+++ b/pkgs/development/compilers/llvm/default.nix
@@ -31,9 +31,9 @@ let
     "18.1.8".officialRelease.sha256 = "sha256-iiZKMRo/WxJaBXct9GdAcAT3cz9d9pnAcO1mmR6oPNE=";
     "19.1.6".officialRelease.sha256 = "sha256-LD4nIjZTSZJtbgW6tZopbTF5Mq0Tenj2gbuPXhtOeUI=";
     "20.0.0-git".gitRelease = {
-      rev = "eb5cda480d2ad81230b2aa3e134e2b603ff90a1c";
-      rev-version = "20.0.0-unstable-2024-11-26";
-      sha256 = "sha256-8VbQINEZZqAIF4egMaNPd3/W3E3QmOXMl7WToftrebg=";
+      rev = "1ef5b987a464611a60e873650726b5e02fda0feb";
+      rev-version = "20.0.0-unstable-2024-12-17";
+      sha256 = "sha256-QCY9z9h3z5gPvwq6bNzAB5xFFStwOCfKh4VnWInhxU4=";
     };
   } // llvmVersions;
 
diff --git a/pkgs/top-level/all-packages.nix b/pkgs/top-level/all-packages.nix
index c616d6aba7d3..a21f709c36bf 100644
--- a/pkgs/top-level/all-packages.nix
+++ b/pkgs/top-level/all-packages.nix
@@ -9017,6 +9017,9 @@ with pkgs;
     else if name == "nblibc" then targetPackages.netbsd.libc or netbsd.libc
     else if name == "wasilibc" then targetPackages.wasilibc or wasilibc
     else if name == "relibc" then targetPackages.relibc or relibc
+    else if name == "llvm" then
+      # Use llvmPackages_git until LLVM 20 is the default.
+      targetPackages.llvmPackages_git.libc or llvmPackages_git.libc
     else throw "Unknown libc ${name}";
 
   libcCross =
diff --git a/pkgs/top-level/release-attrpaths-superset.nix b/pkgs/top-level/release-attrpaths-superset.nix
index 9a89062bd174..c306a64be585 100644
--- a/pkgs/top-level/release-attrpaths-superset.nix
+++ b/pkgs/top-level/release-attrpaths-superset.nix
@@ -50,6 +50,7 @@ let
 
     # cross packagesets
     pkgsLLVM = true;
+    pkgsLLVMLibc = true;
     pkgsMusl = true;
     pkgsStatic = true;
     pkgsCross = true;
diff --git a/pkgs/top-level/release.nix b/pkgs/top-level/release.nix
index 78bdf1491125..97e64f73c510 100644
--- a/pkgs/top-level/release.nix
+++ b/pkgs/top-level/release.nix
@@ -345,6 +345,7 @@ let
       agdaPackages = packagePlatforms pkgs.agdaPackages;
 
       pkgsLLVM.stdenv = [ "x86_64-linux" "aarch64-linux" ];
+      pkgsLLVMLibc.stdenv = [ "x86_64-linux" "aarch64-linux" ];
       pkgsArocc.stdenv = [ "x86_64-linux" "aarch64-linux" ];
       pkgsZig.stdenv = [ "x86_64-linux" "aarch64-linux" ];
       pkgsMusl.stdenv = [ "x86_64-linux" "aarch64-linux" ];
diff --git a/pkgs/top-level/stage.nix b/pkgs/top-level/stage.nix
index 1cedd8dd1845..64bc2308f6da 100644
--- a/pkgs/top-level/stage.nix
+++ b/pkgs/top-level/stage.nix
@@ -98,6 +98,10 @@ let
           or lib.systems.parse.abis.musl;
       });
 
+  makeLLVMParsedPlatform = parsed:
+    (parsed // {
+      abi = lib.systems.parse.abis.llvm;
+    });
 
   stdenvAdapters = self: super:
     let
@@ -207,6 +211,19 @@ let
       };
     };
 
+    pkgsLLVMLibc = nixpkgsFun {
+      overlays = [ (self': super': {
+        pkgsLLVMLibc = super';
+      })] ++ overlays;
+      # Bootstrap a cross stdenv using LLVM libc.
+      # This is currently not possible when compiling natively,
+      # so we don't need to check hostPlatform != buildPlatform.
+      crossSystem = stdenv.hostPlatform // {
+        config = lib.systems.parse.tripleFromSystem (makeLLVMParsedPlatform stdenv.hostPlatform.parsed);
+        libc = "llvm";
+      };
+    };
+
     pkgsArocc = nixpkgsFun {
       overlays = [
         (self': super': {