diff --git a/src/tools/miri/cargo-miri/src/phases.rs b/src/tools/miri/cargo-miri/src/phases.rs
index d74e0c5157d..11e7943ec8f 100644
--- a/src/tools/miri/cargo-miri/src/phases.rs
+++ b/src/tools/miri/cargo-miri/src/phases.rs
@@ -94,7 +94,7 @@ pub fn phase_cargo_miri(mut args: impl Iterator<Item = String>) {
     let target = target.as_ref().unwrap_or(host);
 
     // We always setup.
-    setup(&subcommand, target, &rustc_version, verbose);
+    let miri_sysroot = setup(&subcommand, target, &rustc_version, verbose);
 
     // Invoke actual cargo for the job, but with different flags.
     // We re-use `cargo test` and `cargo run`, which makes target and binary handling very easy but
@@ -159,6 +159,8 @@ pub fn phase_cargo_miri(mut args: impl Iterator<Item = String>) {
     // Forward all further arguments (not consumed by `ArgSplitFlagValue`) to cargo.
     cmd.args(args);
 
+    // Let it know where the Miri sysroot lives.
+    cmd.env("MIRI_SYSROOT", miri_sysroot);
     // Set `RUSTC_WRAPPER` to ourselves.  Cargo will prepend that binary to its usual invocation,
     // i.e., the first argument is `rustc` -- which is what we use in `main` to distinguish
     // the two codepaths. (That extra argument is why we prefer this over setting `RUSTC`.)
diff --git a/src/tools/miri/cargo-miri/src/setup.rs b/src/tools/miri/cargo-miri/src/setup.rs
index 2e4f0a71013..55a97bafbcf 100644
--- a/src/tools/miri/cargo-miri/src/setup.rs
+++ b/src/tools/miri/cargo-miri/src/setup.rs
@@ -13,13 +13,20 @@ use crate::util::*;
 /// Performs the setup required to make `cargo miri` work: Getting a custom-built libstd. Then sets
 /// `MIRI_SYSROOT`. Skipped if `MIRI_SYSROOT` is already set, in which case we expect the user has
 /// done all this already.
-pub fn setup(subcommand: &MiriCommand, target: &str, rustc_version: &VersionMeta, verbose: usize) {
+pub fn setup(
+    subcommand: &MiriCommand,
+    target: &str,
+    rustc_version: &VersionMeta,
+    verbose: usize,
+) -> PathBuf {
     let only_setup = matches!(subcommand, MiriCommand::Setup);
     let ask_user = !only_setup;
     let print_sysroot = only_setup && has_arg_flag("--print-sysroot"); // whether we just print the sysroot path
-    if !only_setup && std::env::var_os("MIRI_SYSROOT").is_some() {
-        // Skip setup step if MIRI_SYSROOT is explicitly set, *unless* we are `cargo miri setup`.
-        return;
+    if !only_setup {
+        if let Some(sysroot) = std::env::var_os("MIRI_SYSROOT") {
+            // Skip setup step if MIRI_SYSROOT is explicitly set, *unless* we are `cargo miri setup`.
+            return sysroot.into();
+        }
     }
 
     // Determine where the rust sources are located.  The env var trumps auto-detection.
@@ -92,6 +99,8 @@ pub fn setup(subcommand: &MiriCommand, target: &str, rustc_version: &VersionMeta
             command.env("RUSTC", &cargo_miri_path);
         }
         command.env("MIRI_CALLED_FROM_SETUP", "1");
+        // Miri expects `MIRI_SYSROOT` to be set when invoked in target mode. Even if that directory is empty.
+        command.env("MIRI_SYSROOT", &sysroot_dir);
         // Make sure there are no other wrappers getting in our way (Cc
         // https://github.com/rust-lang/miri/issues/1421,
         // https://github.com/rust-lang/miri/issues/2429). Looks like setting
@@ -117,8 +126,6 @@ pub fn setup(subcommand: &MiriCommand, target: &str, rustc_version: &VersionMeta
     // the user might have set, which is consistent with normal `cargo build` that does
     // not apply `RUSTFLAGS` to the sysroot either.
     let rustflags = &["-Cdebug-assertions=off", "-Coverflow-checks=on"];
-    // Make sure all target-level Miri invocations know their sysroot.
-    std::env::set_var("MIRI_SYSROOT", &sysroot_dir);
 
     // Do the build.
     if print_sysroot {
@@ -159,4 +166,6 @@ pub fn setup(subcommand: &MiriCommand, target: &str, rustc_version: &VersionMeta
         // Print just the sysroot and nothing else to stdout; this way we do not need any escaping.
         println!("{}", sysroot_dir.display());
     }
+
+    sysroot_dir
 }