diff --git a/cargo/private/cargo_build_script.bzl b/cargo/private/cargo_build_script.bzl
index 7ede86794b..0dbdfdc68f 100644
--- a/cargo/private/cargo_build_script.bzl
+++ b/cargo/private/cargo_build_script.bzl
@@ -365,13 +365,16 @@ def _cargo_build_script_impl(ctx):
         "CARGO_MANIFEST_DIR": manifest_dir,
         "CARGO_PKG_NAME": pkg_name,
         "HOST": toolchain.exec_triple.str,
-        "NUM_JOBS": "1",
         "OPT_LEVEL": compilation_mode_opt_level,
         "RUSTC": toolchain.rustc.path,
         "TARGET": toolchain.target_flag_value,
         # OUT_DIR is set by the runner itself, rather than on the action.
     })
 
+    # Allow the user to override the number of jobs for the build script.
+    if "NUM_JOBS" not in env:
+        env["NUM_JOBS"] = 1
+
     # This isn't exactly right, but Bazel doesn't have exact views of "debug" and "release", so...
     env.update({
         "DEBUG": {"dbg": "true", "fastbuild": "true", "opt": "false"}.get(ctx.var["COMPILATION_MODE"], "true"),