diff --git a/tests/msc3389/main_test.go b/tests/msc3389/main_test.go
new file mode 100644
index 00000000..09db65b7
--- /dev/null
+++ b/tests/msc3389/main_test.go
@@ -0,0 +1,11 @@
+package tests
+
+import (
+	"testing"
+
+	"github.com/matrix-org/complement"
+)
+
+func TestMain(m *testing.M) {
+	complement.TestMain(m, "msc3389")
+}
diff --git a/tests/msc3389/msc3389_test.go b/tests/msc3389/msc3389_test.go
new file mode 100644
index 00000000..a523b7d6
--- /dev/null
+++ b/tests/msc3389/msc3389_test.go
@@ -0,0 +1,155 @@
+package tests
+
+import (
+	"testing"
+
+	"github.com/matrix-org/complement"
+	"github.com/matrix-org/complement/b"
+	"github.com/matrix-org/complement/client"
+	"github.com/matrix-org/complement/helpers"
+	"github.com/matrix-org/complement/match"
+	"github.com/matrix-org/complement/must"
+)
+
+// The room version that enables MSC3389 ("Redaction changes for events with a relation").
+const msc3389RoomVersion = "org.matrix.msc3389.10"
+
+// sendRelatedEvent sends a reaction (m.annotation) targeting parentID, with an extra
+// `key` field that should not survive redaction, and returns the event ID of the reaction.
+func sendRelatedEvent(t *testing.T, alice *client.CSAPI, roomID, parentID string) string {
+	return alice.SendEventSynced(t, roomID, b.Event{
+		Type: "m.reaction",
+		Content: map[string]interface{}{
+			"m.relates_to": map[string]interface{}{
+				"rel_type": "m.annotation",
+				"event_id": parentID,
+				"key":      "👍",
+			},
+		},
+		Sender: alice.UserID,
+	})
+}
+
+// TestMSC3389RedactionPreservesRelation checks that, in a room version that implements
+// MSC3389, redacting an event with an `m.relates_to` field keeps `rel_type` and `event_id`
+// but strips every other key (such as the reaction `key`).
+func TestMSC3389RedactionPreservesRelation(t *testing.T) {
+	deployment := complement.Deploy(t, 1)
+	defer deployment.Destroy(t)
+
+	alice := deployment.Register(t, "hs1", helpers.RegistrationOpts{})
+
+	// Alice creates a room in the MSC3389 unstable room version.
+	roomID := alice.MustCreateRoom(t, map[string]interface{}{
+		"preset":       "public_chat",
+		"room_version": msc3389RoomVersion,
+	})
+
+	parentID := alice.SendEventSynced(t, roomID, b.Event{
+		Type: "m.room.message",
+		Content: map[string]interface{}{
+			"msgtype": "m.text",
+			"body":    "root",
+		},
+		Sender: alice.UserID,
+	})
+
+	reactionID := sendRelatedEvent(t, alice, roomID, parentID)
+
+	// Verify the original event is fully intact before redaction.
+	eventJsonBefore := alice.MustGetEvent(t, roomID, reactionID)
+	must.MatchGJSON(t, eventJsonBefore,
+		match.JSONKeyEqual("content.m.relates_to.rel_type", "m.annotation"),
+		match.JSONKeyEqual("content.m.relates_to.event_id", parentID),
+		match.JSONKeyEqual("content.m.relates_to.key", "👍"),
+	)
+
+	alice.MustSendRedaction(t, roomID, map[string]interface{}{}, reactionID)
+
+	// After redaction the `m.relates_to` object is retained, but only `rel_type` and
+	// `event_id` survive; the `key` is removed.
+	eventJsonAfter := alice.MustGetEvent(t, roomID, reactionID)
+	must.MatchGJSON(t, eventJsonAfter,
+		match.JSONKeyEqual("content.m.relates_to.rel_type", "m.annotation"),
+		match.JSONKeyEqual("content.m.relates_to.event_id", parentID),
+		match.JSONKeyMissing("content.m.relates_to.key"),
+	)
+}
+
+// TestMSC3389RedactionStripsRelationInOlderVersions checks the contrast: in a room version
+// that predates MSC3389 (e.g. v9), redaction removes the whole `m.relates_to` field.
+func TestMSC3389RedactionStripsRelationInOlderVersions(t *testing.T) {
+	deployment := complement.Deploy(t, 1)
+	defer deployment.Destroy(t)
+
+	alice := deployment.Register(t, "hs1", helpers.RegistrationOpts{})
+
+	roomID := alice.MustCreateRoom(t, map[string]interface{}{
+		"preset":       "public_chat",
+		"room_version": "9",
+	})
+
+	parentID := alice.SendEventSynced(t, roomID, b.Event{
+		Type: "m.room.message",
+		Content: map[string]interface{}{
+			"msgtype": "m.text",
+			"body":    "root",
+		},
+		Sender: alice.UserID,
+	})
+
+	reactionID := sendRelatedEvent(t, alice, roomID, parentID)
+
+	alice.MustSendRedaction(t, roomID, map[string]interface{}{}, reactionID)
+
+	// In room v9 the whole `m.relates_to` is stripped, leaving no content.
+	eventJsonAfter := alice.MustGetEvent(t, roomID, reactionID)
+	must.MatchGJSON(t, eventJsonAfter,
+		match.JSONKeyMissing("content.m.relates_to"),
+	)
+}
+
+// TestMSC3389RedactionPreservesPlainRelation checks that an `m.relates_to` containing only
+// `rel_type` and `event_id` (with no extra keys) survives redaction unchanged.
+func TestMSC3389RedactionPreservesPlainRelation(t *testing.T) {
+	deployment := complement.Deploy(t, 1)
+	defer deployment.Destroy(t)
+
+	alice := deployment.Register(t, "hs1", helpers.RegistrationOpts{})
+
+	roomID := alice.MustCreateRoom(t, map[string]interface{}{
+		"preset":       "public_chat",
+		"room_version": msc3389RoomVersion,
+	})
+
+	parentID := alice.SendEventSynced(t, roomID, b.Event{
+		Type: "m.room.message",
+		Content: map[string]interface{}{
+			"msgtype": "m.text",
+			"body":    "root",
+		},
+		Sender: alice.UserID,
+	})
+
+	// A threaded reply has no extra `m.relates_to` keys beyond rel_type + event_id.
+	replyID := alice.SendEventSynced(t, roomID, b.Event{
+		Type: "m.room.message",
+		Content: map[string]interface{}{
+			"msgtype": "m.text",
+			"body":    "reply",
+			"m.relates_to": map[string]interface{}{
+				"rel_type": "m.thread",
+				"event_id": parentID,
+			},
+		},
+		Sender: alice.UserID,
+	})
+
+	alice.MustSendRedaction(t, roomID, map[string]interface{}{}, replyID)
+
+	eventJsonAfter := alice.MustGetEvent(t, roomID, replyID)
+	must.MatchGJSON(t, eventJsonAfter,
+		match.JSONKeyEqual("content.m.relates_to.rel_type", "m.thread"),
+		match.JSONKeyEqual("content.m.relates_to.event_id", parentID),
+	)
+}