diff --git a/library/test/src/lib.rs b/library/test/src/lib.rs
index 9c5bb8957b548..6570cadef8498 100644
--- a/library/test/src/lib.rs
+++ b/library/test/src/lib.rs
@@ -470,7 +470,14 @@ pub fn run_test(
         let supports_threads = !cfg!(target_os = "emscripten") && !cfg!(target_arch = "wasm32");
         if concurrency == Concurrent::Yes && supports_threads {
             let cfg = thread::Builder::new().name(name.as_slice().to_owned());
-            cfg.spawn(runtest).unwrap();
+            if let Err(err) = cfg.spawn(runtest) {
+                if err.kind() == io::ErrorKind::WouldBlock {
+                    // If we're at the thread limit, just run all tests in a single thread.
+                    runtest();
+                } else {
+                    panic!("spawning a test thread failed: {}", err);
+                }
+            }
         } else {
             runtest();
         }