diff --git a/lib/internal/test_runner/test.js b/lib/internal/test_runner/test.js
index f7a07a100c91a4..704cbd7702ed78 100644
--- a/lib/internal/test_runner/test.js
+++ b/lib/internal/test_runner/test.js
@@ -43,7 +43,7 @@ const noop = FunctionPrototype;
 const isTestRunner = getOptionValue('--test');
 const testOnlyFlag = !isTestRunner && getOptionValue('--test-only');
 // TODO(cjihrig): Use uv_available_parallelism() once it lands.
-const rootConcurrency = isTestRunner ? cpus().length : 1;
+const rootConcurrency = isTestRunner ? cpus().length - 1 : 1;
 const subTestConcurrency = getOptionValue('--test-concurrency');
 
 function testTimeout(promise, timeout) {