diff --git a/src/tools/compiletest/src/runtest.rs b/src/tools/compiletest/src/runtest.rs
index e8d0ebd40b0..672779325ae 100644
--- a/src/tools/compiletest/src/runtest.rs
+++ b/src/tools/compiletest/src/runtest.rs
@@ -4022,8 +4022,11 @@ impl<'test> TestCx<'test> {
     }
 
     fn normalize_output(&self, output: &str, custom_rules: &[(String, String)]) -> String {
+        let rflags = self.props.run_flags.as_ref();
         let cflags = self.props.compile_flags.join(" ");
-        let json = cflags.contains("--error-format json")
+        let json = rflags
+            .map_or(false, |s| s.contains("--format json") || s.contains("--format=json"))
+            || cflags.contains("--error-format json")
             || cflags.contains("--error-format pretty-json")
             || cflags.contains("--error-format=json")
             || cflags.contains("--error-format=pretty-json")