diff --git a/tests/msc3391_test.go b/tests/msc3391_test.go
new file mode 100644
index 00000000..e3613e95
--- /dev/null
+++ b/tests/msc3391_test.go
@@ -0,0 +1,274 @@
+//go:build msc3391
+// +build msc3391
+
+// This file contains tests for deleting account data as
+// defined by MSC3391, which you can read here:
+// https://github.com/matrix-org/matrix-doc/pull/3391
+
+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/match"
+	"github.com/matrix-org/complement/internal/must"
+
+	"github.com/tidwall/gjson"
+)
+
+const testAccountDataType = "org.example.test"
+
+var testAccountDataContent = map[string]interface{}{"test_data": 1}
+
+func TestRemovingAccountData(t *testing.T) {
+	deployment := Deploy(t, b.BlueprintAlice)
+	defer deployment.Destroy(t)
+
+	// Create a user to manipulate the account data of
+	aliceUserID := "@alice:hs1"
+	alice := deployment.Client(t, "hs1", aliceUserID)
+
+	// And create a room with that user where we can store some room account data
+	roomID := alice.CreateRoom(t, map[string]interface{}{})
+
+	for _, httpMethod := range []string{"PUT", "DELETE"} {
+		t.Run(fmt.Sprintf("Deleting a user's account data via %s works", httpMethod), func(t *testing.T) {
+			createAndDeleteAccountData(t, alice, httpMethod == "DELETE", nil)
+		})
+		t.Run("Deleting a user's room account data via ", func(t *testing.T) {
+			createAndDeleteAccountData(t, alice, httpMethod == "DELETE", &roomID)
+		})
+	}
+}
+
+func createAndDeleteAccountData(t *testing.T, c *client.CSAPI, viaDelete bool, roomID *string) {
+	// Create the account data and check that it has been created successfully
+	createAccountData(t, c, roomID)
+
+	// Delete the account data and check that it was deleted successfully
+	deleteAccountData(t, c, viaDelete, roomID)
+}
+
+// createAccountData creates some account data for a user or a room, and checks that it was
+// created successfully by both querying the data afterwards, and ensuring it appears down /sync.
+func createAccountData(t *testing.T, c *client.CSAPI, roomID *string) {
+	// a function to check that the content of a user or account data object
+	// matches our test content.
+	checkAccountData := func(r gjson.Result) bool {
+		// Only listen for our test type
+		if r.Get("type").Str != testAccountDataType {
+			return false
+		}
+		content := r.Get("content")
+
+		// Ensure the content of this account data type is as we expect
+		return match.JSONDeepEqual([]byte(content.Raw), testAccountDataContent)
+	}
+
+	// Retrieve a sync token for this user
+	_, nextBatchToken := c.MustSync(
+		t,
+		client.SyncReq{},
+	)
+
+	// Set and check the account data
+	if roomID != nil {
+		// Create room account data
+		c.SetRoomAccountData(t, *roomID, testAccountDataType, testAccountDataContent)
+
+		// Wait for the account data to appear down /sync
+		c.MustSyncUntil(
+			t,
+			client.SyncReq{
+				Since: nextBatchToken,
+			},
+			client.SyncRoomAccountDataHas(*roomID, checkAccountData),
+		)
+
+		// Also check the account data content by querying the appropriate endpoint
+		res := c.GetRoomAccountData(t, *roomID, testAccountDataType)
+		must.MatchResponse(t, res, match.HTTPResponse{
+			JSON: []match.JSON{
+				func(body []byte) error {
+					if !match.JSONDeepEqual(body, testAccountDataContent) {
+						return fmt.Errorf(
+							"Expected %s for room account data content when, got '%s'",
+							testAccountDataType,
+							string(body),
+						)
+					}
+
+					return nil
+				},
+			},
+		})
+	} else {
+		// Create user account data
+		c.SetGlobalAccountData(t, testAccountDataType, testAccountDataContent)
+
+		// Wait for the account data to appear down /sync
+		c.MustSyncUntil(
+			t,
+			client.SyncReq{
+				Since: nextBatchToken,
+			},
+			client.SyncGlobalAccountDataHas(checkAccountData),
+		)
+
+		// Also check the account data content by querying the appropriate endpoint
+		res := c.GetGlobalAccountData(t, testAccountDataType)
+		must.MatchResponse(t, res, match.HTTPResponse{
+			JSON: []match.JSON{
+				func(body []byte) error {
+					if !match.JSONDeepEqual(body, testAccountDataContent) {
+						return fmt.Errorf(
+							"Expected %s for room account data content when, got '%s'",
+							testAccountDataType,
+							string(body),
+						)
+					}
+
+					return nil
+				},
+			},
+		})
+	}
+}
+
+// deleteAccountData removes account data for a user or room.
+//
+// If viaDelete is true, a request is made to the DELETE endpoint for user or
+// room account data. Otherwise, the PUT method is used with an empty content
+// dictionary instead. MSC3391 specifies that a PUT with an empty content body
+// is functionally equivalent to deleting an account data type directly.
+//
+// If roomID is not nil, room account data for the given room ID will be removed.
+// Otherwise, account data from the user will be removed instead.
+func deleteAccountData(t *testing.T, c *client.CSAPI, viaDelete bool, roomID *string) {
+	// a function to check that the content of a user or account data object
+	// matches our test content.
+	checkEmptyAccountData := func(r gjson.Result) bool {
+		// Only listen for our test type
+		if r.Get("type").Str != testAccountDataType {
+			return false
+		}
+		content := r.Get("content")
+
+		// Ensure the content of this account data type is an empty map.
+		// This means that it has been deleted.
+		return match.JSONDeepEqual([]byte(content.Raw), map[string]interface{}{})
+	}
+
+	// a function that checks that a given account data event type is not present
+	checkAccountDataTypeNotPresent := func(r gjson.Result) error {
+		// If we see our test type, return a failure
+		if r.Get("type").Str == testAccountDataType {
+			return fmt.Errorf(
+				"Found unexpected account data type '%s' in sync response",
+				testAccountDataType,
+			)
+		}
+
+		// We did not see our test type.
+		return nil
+	}
+
+	// Retrieve a sync token for this user
+	_, nextBatchToken := c.MustSync(
+		t,
+		client.SyncReq{},
+	)
+
+	if roomID != nil {
+		// Delete room account data
+		if viaDelete {
+			// Delete via the DELETE method
+			c.MustDoFunc(
+				t,
+				"DELETE",
+				[]string{"_matrix", "client", "unstable", "org.matrix.msc3391", "user", c.UserID, "rooms", *roomID, "account_data", testAccountDataType},
+			)
+		} else {
+			// Delete via the PUT method. PUT'ing with an empty dictionary will delete
+			// the account data type for this room.
+			c.SetRoomAccountData(t, *roomID, testAccountDataType, map[string]interface{}{})
+		}
+
+		// Check that the content of the room account data for this type
+		// has been set to an empty dictionary.
+		c.MustSyncUntil(
+			t,
+			client.SyncReq{
+				Since: nextBatchToken,
+			},
+			client.SyncRoomAccountDataHas(*roomID, checkEmptyAccountData),
+		)
+
+		// Also check the account data item is no longer found
+		res := c.DoFunc(t, "GET", []string{"_matrix", "client", "v3", "user", c.UserID, "room", *roomID, "account_data", testAccountDataType})
+		must.MatchResponse(t, res, match.HTTPResponse{
+			StatusCode: 404,
+		})
+
+		// Finally, check that the account data item does not appear at all in an initial sync
+		initialSyncResponse, _ := c.MustSync(
+			t,
+			client.SyncReq{},
+		)
+		must.MatchGJSON(
+			t,
+			initialSyncResponse,
+			match.JSONArrayEach(
+				fmt.Sprintf("rooms.join.%s.account_data.events", *roomID),
+				checkAccountDataTypeNotPresent,
+			),
+		)
+	} else {
+		// Delete user account data
+		if viaDelete {
+			// Delete via the DELETE method
+			c.MustDoFunc(
+				t,
+				"DELETE",
+				[]string{"_matrix", "client", "unstable", "org.matrix.msc3391", "user", c.UserID, "account_data", testAccountDataType},
+			)
+		} else {
+			// Delete via the PUT method. PUT'ing with an empty dictionary will delete
+			// the account data type for this user.
+			c.SetGlobalAccountData(t, testAccountDataType, map[string]interface{}{})
+		}
+
+		// Check that the content of the user account data for this type
+		// has been set to an empty dictionary.
+		c.MustSyncUntil(
+			t,
+			client.SyncReq{
+				Since: nextBatchToken,
+			},
+			client.SyncGlobalAccountDataHas(checkEmptyAccountData),
+		)
+
+		// Also check the account data item is no longer found
+		res := c.DoFunc(t, "GET", []string{"_matrix", "client", "v3", "user", c.UserID, "account_data", testAccountDataType})
+		must.MatchResponse(t, res, match.HTTPResponse{
+			StatusCode: 404,
+		})
+
+		// Finally, check that the account data item does not appear at all in an initial sync
+		initialSyncResponse, _ := c.MustSync(
+			t,
+			client.SyncReq{},
+		)
+		must.MatchGJSON(
+			t,
+			initialSyncResponse,
+			match.JSONArrayEach(
+				"account_data.events",
+				checkAccountDataTypeNotPresent,
+			),
+		)
+	}
+}