diff --git a/tests/msc3970_test.go b/tests/msc3970_test.go
new file mode 100644
index 00000000..548ac1d6
--- /dev/null
+++ b/tests/msc3970_test.go
@@ -0,0 +1,109 @@
+//go:build msc3970
+// +build msc3970
+
+// This files contains tests for MSC3970, which changes the scope
+// of transaction IDs to be per-device instead of per-access token.
+// https://github.com/matrix-org/matrix-spec-proposals/pull/3970
+
+package tests
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/matrix-org/complement/internal/b"
+	"github.com/matrix-org/complement/internal/client"
+	"github.com/matrix-org/complement/internal/must"
+	"github.com/tidwall/gjson"
+)
+
+func mustHaveTransactionID(t *testing.T, roomID, eventID, expectedTxnId string) client.SyncCheckOpt {
+	return client.SyncTimelineHas(roomID, func(r gjson.Result) bool {
+		if r.Get("event_id").Str == eventID {
+			unsignedTxnId := r.Get("unsigned.transaction_id")
+			if !unsignedTxnId.Exists() {
+				t.Fatalf("Event %s in room %s should have a 'unsigned.transaction_id', but it did not", eventID, roomID)
+			}
+
+			must.EqualStr(t, unsignedTxnId.Str, expectedTxnId, fmt.Sprintf("Event %s in room %s should have a 'unsigned.transaction_id'", eventID, roomID))
+
+			return true
+		}
+
+		return false
+	})
+}
+
+// TestTxnScopeOnLocalEchoMSC3970 checks that the transaction IDs are scoped to the device,
+// and not just the access token, as per MSC3970.
+func TestTxnScopeOnLocalEchoMSC3970(t *testing.T) {
+	deployment := Deploy(t, b.BlueprintCleanHS)
+	defer deployment.Destroy(t)
+
+	deployment.RegisterUser(t, "hs1", "alice", "password", false)
+
+	// Create a first client, which allocates a device ID.
+	c1 := deployment.Client(t, "hs1", "")
+	c1.UserID, c1.AccessToken, c1.DeviceID = c1.LoginUser(t, "alice", "password")
+
+	// Create a room where we can send events.
+	roomID := c1.CreateRoom(t, map[string]interface{}{})
+
+	txnId := "abdefgh"
+	// Let's send an event, and wait for it to appear in the timeline.
+	eventID := c1.SendEventUnsyncedWithTxnID(t, roomID, b.Event{
+		Type: "m.room.message",
+		Content: map[string]interface{}{
+			"msgtype": "m.text",
+			"body":    "first",
+		},
+	}, txnId)
+
+	// When syncing, we should find the event and it should have a transaction ID on the first client.
+	c1.MustSyncUntil(t, client.SyncReq{}, mustHaveTransactionID(t, roomID, eventID, txnId))
+
+	// Create a second client, inheriting the first device ID.
+	c2 := deployment.Client(t, "hs1", "")
+	c2.UserID, c2.AccessToken, _ = c2.LoginUser(t, "alice", "password", client.WithDeviceID(c1.DeviceID))
+	c2.DeviceID = c1.DeviceID
+
+	// When syncing, we should find the event and it should *not* have a transaction ID on the second client.
+	c2.MustSyncUntil(t, client.SyncReq{}, mustHaveTransactionID(t, roomID, eventID, txnId))
+}
+
+// TestTxnIdempotencyScopedToClientDeviceMSC3970 tests that transaction IDs are scoped to a device
+func TestTxnIdempotencyScopedToDeviceMSC3970(t *testing.T) {
+	deployment := Deploy(t, b.BlueprintCleanHS)
+	defer deployment.Destroy(t)
+
+	deployment.RegisterUser(t, "hs1", "alice", "password", false)
+
+	// Create a first client, which allocates a device ID.
+	c1 := deployment.Client(t, "hs1", "")
+	c1.UserID, c1.AccessToken, c1.DeviceID = c1.LoginUser(t, "alice", "password")
+
+	// Create a room where we can send events.
+	roomID := c1.CreateRoom(t, map[string]interface{}{})
+
+	txnId := "abcdef"
+	event := b.Event{
+		Type: "m.room.message",
+		Content: map[string]interface{}{
+			"msgtype": "m.text",
+			"body":    "foo",
+		},
+	}
+	// send an event with set txnId
+	eventID1 := c1.SendEventUnsyncedWithTxnID(t, roomID, event, txnId)
+
+	// Create a second client, inheriting the first device ID.
+	c2 := deployment.Client(t, "hs1", "")
+	c2.UserID, c2.AccessToken, _ = c2.LoginUser(t, "alice", "password", client.WithDeviceID(c1.DeviceID))
+	c2.DeviceID = c1.DeviceID
+
+	// send another event with the same txnId
+	eventID2 := c2.SendEventUnsyncedWithTxnID(t, roomID, event, txnId)
+
+	// the two events should have different event IDs as they came from different clients
+	must.EqualStr(t, eventID2, eventID1, "Expected eventID1 and eventID2 to be equal from two clients sharing the same device ID")
+}