diff --git a/src/testing/testing.go b/src/testing/testing.go
index 2081365abf3..c33a997372c 100644
--- a/src/testing/testing.go
+++ b/src/testing/testing.go
@@ -293,7 +293,8 @@ var _ TB = (*B)(nil)
 // may be called simultaneously from multiple goroutines.
 type T struct {
 	common
-	name          string    // Name of test.
+	name          string // Name of test.
+	isParallel    bool
 	startParallel chan bool // Parallel tests will wait on this.
 }
 
@@ -430,6 +431,10 @@ func (t *T) Parallel() {
 	// We don't want to include the time we spend waiting for serial tests
 	// in the test duration. Record the elapsed time thus far and reset the
 	// timer afterwards.
+	if t.isParallel {
+		panic("testing: t.Parallel called multiple times")
+	}
+	t.isParallel = true
 	t.duration += time.Since(t.start)
 	t.signal <- (*T)(nil) // Release main testing loop
 	<-t.startParallel     // Wait for serial tests to finish