diff --git a/tools/http2_interop/s6.5.go b/tools/http2_interop/s6.5.go
index 32468abe831..4295c46f73a 100644
--- a/tools/http2_interop/s6.5.go
+++ b/tools/http2_interop/s6.5.go
@@ -1,6 +1,7 @@
 package http2interop
 
 import (
+	"fmt"
 	"time"
 )
 
@@ -30,3 +31,60 @@ func testSmallMaxFrameSize(ctx *HTTP2InteropCtx) error {
 
 	return nil
 }
+
+// Section 6.5.3 says all settings frames must be acked.
+func testAllSettingsFramesAcked(ctx *HTTP2InteropCtx) error {
+	conn, err := connect(ctx)
+	if err != nil {
+		return err
+	}
+	defer conn.Close()
+	conn.SetDeadline(time.Now().Add(defaultTimeout))
+
+	sf := &SettingsFrame{}
+	if err := http2Connect(conn, sf); err != nil {
+		return err
+	}
+
+	// The spec says "The values in the SETTINGS frame MUST be processed in the order they
+	// appear. [...] Once all values have been processed, the recipient MUST immediately
+	// emit a SETTINGS frame with the ACK flag set."  From my understanding, processing all
+	// of no values warrants an ack per frame.
+	for i := 0; i < 10; i++ {
+		if err := streamFrame(conn, sf); err != nil {
+			return err
+		}
+	}
+
+	var settingsFramesReceived = 0
+	// The server by default sends a settings frame as part of the handshake, and another
+	// after the receipt of the initial settings frame as part of our conneection preface.
+	// This means we expected 1 + 1 + 10 = 12 settings frames in return, with all but the
+	// first having the ack bit.
+	for settingsFramesReceived < 12 {
+		f, err := parseFrame(conn)
+		if err != nil {
+			return err
+		}
+
+		// Other frames come down the wire too, including window update.  Just ignore those.
+		if f, ok := f.(*SettingsFrame); ok {
+			settingsFramesReceived += 1
+			if settingsFramesReceived == 1 {
+				if f.Header.Flags&SETTINGS_FLAG_ACK > 0 {
+					return fmt.Errorf("settings frame should not have used ack: %v")
+				}
+				continue
+			}
+
+			if f.Header.Flags&SETTINGS_FLAG_ACK == 0 {
+				return fmt.Errorf("settings frame should have used ack: %v", f)
+			}
+			if len(f.Params) != 0 {
+				return fmt.Errorf("settings ack cannot have params: %v", f)
+			}
+		}
+	}
+
+	return nil
+}
diff --git a/tools/http2_interop/s6.5_test.go b/tools/http2_interop/s6.5_test.go
index 9dadd4e699c..063fd5664c8 100644
--- a/tools/http2_interop/s6.5_test.go
+++ b/tools/http2_interop/s6.5_test.go
@@ -13,3 +13,14 @@ func TestSoonSmallMaxFrameSize(t *testing.T) {
 	err := testSmallMaxFrameSize(ctx)
 	matchError(t, err, "Got goaway frame")
 }
+
+func TestSoonAllSettingsFramesAcked(t *testing.T) {
+	defer Report(t)
+	if *testCase != "framing" {
+		t.SkipNow()
+	}
+	ctx := InteropCtx(t)
+	if err := testAllSettingsFramesAcked(ctx); err != nil {
+		t.Fatal(err)
+	}
+}
diff --git a/tools/http2_interop/settings.go b/tools/http2_interop/settings.go
index 97914d960fa..544cec01ee7 100644
--- a/tools/http2_interop/settings.go
+++ b/tools/http2_interop/settings.go
@@ -26,6 +26,10 @@ const (
 	SettingsMaxHeaderListSize    SettingsIdentifier = 6
 )
 
+const (
+	SETTINGS_FLAG_ACK byte = 0x01
+)
+
 func (si SettingsIdentifier) String() string {
 	switch si {
 	case SettingsHeaderTableSize: