diff --git a/src/net/http/httputil/reverseproxy.go b/src/net/http/httputil/reverseproxy.go
index 488db973133..fd78d45602d 100644
--- a/src/net/http/httputil/reverseproxy.go
+++ b/src/net/http/httputil/reverseproxy.go
@@ -233,7 +233,8 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 
 	// The "Trailer" header isn't included in the Transport's response,
 	// at least for *http.Transport. Build it up from Trailer.
-	if len(res.Trailer) > 0 {
+	announcedTrailers := len(res.Trailer)
+	if announcedTrailers > 0 {
 		trailerKeys := make([]string, 0, len(res.Trailer))
 		for k := range res.Trailer {
 			trailerKeys = append(trailerKeys, k)
@@ -252,7 +253,18 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 	}
 	p.copyResponse(rw, res.Body)
 	res.Body.Close() // close now, instead of defer, to populate res.Trailer
-	copyHeader(rw.Header(), res.Trailer)
+
+	if len(res.Trailer) == announcedTrailers {
+		copyHeader(rw.Header(), res.Trailer)
+		return
+	}
+
+	for k, vv := range res.Trailer {
+		k = http.TrailerPrefix + k
+		for _, v := range vv {
+			rw.Header().Add(k, v)
+		}
+	}
 }
 
 func (p *ReverseProxy) copyResponse(dst io.Writer, src io.Reader) {
diff --git a/src/net/http/httputil/reverseproxy_test.go b/src/net/http/httputil/reverseproxy_test.go
index 008e4e717fb..04ac6b40591 100644
--- a/src/net/http/httputil/reverseproxy_test.go
+++ b/src/net/http/httputil/reverseproxy_test.go
@@ -69,6 +69,7 @@ func TestReverseProxy(t *testing.T) {
 		w.WriteHeader(backendStatus)
 		w.Write([]byte(backendResponse))
 		w.Header().Set("X-Trailer", "trailer_value")
+		w.Header().Set(http.TrailerPrefix+"X-Unannounced-Trailer", "unannounced_trailer_value")
 	}))
 	defer backend.Close()
 	backendURL, err := url.Parse(backend.URL)
@@ -122,6 +123,9 @@ func TestReverseProxy(t *testing.T) {
 	if g, e := res.Trailer.Get("X-Trailer"), "trailer_value"; g != e {
 		t.Errorf("Trailer(X-Trailer) = %q ; want %q", g, e)
 	}
+	if g, e := res.Trailer.Get("X-Unannounced-Trailer"), "unannounced_trailer_value"; g != e {
+		t.Errorf("Trailer(X-Unannounced-Trailer) = %q ; want %q", g, e)
+	}
 
 	// Test that a backend failing to be reached or one which doesn't return
 	// a response results in a StatusBadGateway.