diff --git a/prometheus/promhttp/http.go b/prometheus/promhttp/http.go
index 2e0b9a8..e598e66 100644
--- a/prometheus/promhttp/http.go
+++ b/prometheus/promhttp/http.go
@@ -203,8 +203,10 @@ func HandlerForTransactional(reg prometheus.TransactionalGatherer, opts HandlerO
 
 		defer closeWriter()
 
-		rsp.Header().Set(contentEncodingHeader, encodingHeader)
-
+		// Set Content-Encoding only when data is compressed
+		if encodingHeader != string(Identity) {
+			rsp.Header().Set(contentEncodingHeader, encodingHeader)
+		}
 		enc := expfmt.NewEncoder(w, contentType)
 
 		// handleError handles the error according to opts.ErrorHandling
diff --git a/prometheus/promhttp/http_test.go b/prometheus/promhttp/http_test.go
index 0ed8fe3..3ad2d1d 100644
--- a/prometheus/promhttp/http_test.go
+++ b/prometheus/promhttp/http_test.go
@@ -267,7 +267,7 @@ func TestInstrumentMetricHandler(t *testing.T) {
 		t.Errorf("got HTTP status code %d, want %d", got, want)
 	}
 
-	if got, want := writer.Header().Get(contentEncodingHeader), string(Identity); got != want {
+	if got, want := writer.Header().Get(contentEncodingHeader), ""; got != want {
 		t.Errorf("got HTTP content encoding header %s, want %s", got, want)
 	}