diff --git a/tests/csapi/room_relations_test.go b/tests/csapi/room_relations_test.go
new file mode 100644
index 00000000..ee08105b
--- /dev/null
+++ b/tests/csapi/room_relations_test.go
@@ -0,0 +1,114 @@
+package csapi_tests
+
+import (
+	"net/http"
+	"testing"
+
+	"github.com/tidwall/gjson"
+
+	"github.com/matrix-org/complement/internal/b"
+	"github.com/matrix-org/complement/internal/client"
+	"github.com/matrix-org/complement/internal/match"
+	"github.com/matrix-org/complement/internal/must"
+	"github.com/matrix-org/complement/runtime"
+)
+
+func TestRelations(t *testing.T) {
+	runtime.SkipIf(t, runtime.Dendrite) // not supported
+	deployment := Deploy(t, b.BlueprintAlice)
+	defer deployment.Destroy(t)
+
+	alice := deployment.Client(t, "hs1", "@alice:hs1")
+
+	roomID := alice.CreateRoom(t, map[string]interface{}{"preset": "public_chat"})
+
+	const testMessage = "TestSendAndFetchMessage"
+
+	_, token := alice.MustSync(t, client.SyncReq{})
+
+	res := alice.MustDoFunc(t, "PUT", []string{"_matrix", "client", "v3", "rooms", roomID, "send", "m.room.message", "txn-1"}, client.WithJSONBody(t, map[string]interface{}{
+		"msgtype": "m.text",
+		"body":    "root",
+	}))
+	rootEventID := client.GetJSONFieldStr(t, client.ParseJSON(t, res), "event_id")
+
+	// Send a few related messages.
+	res = alice.MustDoFunc(t, "PUT", []string{"_matrix", "client", "v3", "rooms", roomID, "send", "m.room.message", "txn-2"}, client.WithJSONBody(t, map[string]interface{}{
+		"msgtype": "m.text",
+		"body":    "reply",
+		"m.relates_to": map[string]interface{}{
+			"event_id": rootEventID,
+			"rel_type": "m.thread",
+		},
+	}))
+	threadEventID := client.GetJSONFieldStr(t, client.ParseJSON(t, res), "event_id")
+
+	res = alice.MustDoFunc(t, "PUT", []string{"_matrix", "client", "v3", "rooms", roomID, "send", "m.dummy", "txn-3"}, client.WithJSONBody(t, map[string]interface{}{
+		"m.relates_to": map[string]interface{}{
+			"event_id": rootEventID,
+			"rel_type": "m.thread",
+		},
+	}))
+	dummyEventID := client.GetJSONFieldStr(t, client.ParseJSON(t, res), "event_id")
+
+	res = alice.MustDoFunc(t, "PUT", []string{"_matrix", "client", "v3", "rooms", roomID, "send", "m.room.message", "txn-4"}, client.WithJSONBody(t, map[string]interface{}{
+		"msgtype": "m.text",
+		"body":    "* edited root",
+		"m.new_content": map[string]interface{}{
+			"msgtype": "m.text",
+			"body":    "edited root",
+		},
+		"m.relates_to": map[string]interface{}{
+			"event_id": rootEventID,
+			"rel_type": "m.replace",
+		},
+	}))
+	editEventID := client.GetJSONFieldStr(t, client.ParseJSON(t, res), "event_id")
+
+	// sync until the server has processed it
+	alice.MustSyncUntil(t, client.SyncReq{Since: token}, client.SyncTimelineHas(roomID, func(r gjson.Result) bool {
+		return r.Get("event_id").Str == editEventID
+	}))
+
+	// Request the relations.
+	res = alice.MustDoFunc(t, "GET", []string{"_matrix", "client", "v1", "rooms", roomID, "relations", rootEventID})
+	must.MatchResponse(t, res, match.HTTPResponse{
+		StatusCode: http.StatusOK,
+		JSON: []match.JSON{
+			match.JSONCheckOff("chunk", []interface{}{
+				threadEventID, dummyEventID, editEventID,
+			}, func(r gjson.Result) interface{} {
+				return r.Get("event_id").Str
+			}, nil,
+			),
+		},
+	})
+
+	// Also test filtering by the relation type.
+	res = alice.MustDoFunc(t, "GET", []string{"_matrix", "client", "v1", "rooms", roomID, "relations", rootEventID, "m.thread"})
+	must.MatchResponse(t, res, match.HTTPResponse{
+		StatusCode: http.StatusOK,
+		JSON: []match.JSON{
+			match.JSONCheckOff("chunk", []interface{}{
+				threadEventID, dummyEventID,
+			}, func(r gjson.Result) interface{} {
+				return r.Get("event_id").Str
+			}, nil,
+			),
+		},
+	})
+
+	// And test filtering by relation type + event type.
+	res = alice.MustDoFunc(t, "GET", []string{"_matrix", "client", "v1", "rooms", roomID, "relations", rootEventID, "m.thread", "m.room.message"})
+	must.MatchResponse(t, res, match.HTTPResponse{
+		StatusCode: http.StatusOK,
+		JSON: []match.JSON{
+			match.JSONCheckOff("chunk", []interface{}{
+				threadEventID,
+			}, func(r gjson.Result) interface{} {
+				return r.Get("event_id").Str
+			}, nil,
+			),
+		},
+	})
+}