diff --git a/changelog.d/8823.bugfix b/changelog.d/8823.bugfix
new file mode 100644
index 0000000000..74af1c20b6
--- /dev/null
+++ b/changelog.d/8823.bugfix
@@ -0,0 +1 @@
+Fix `register_new_matrix_user` failing with "Bad Request" when trailing slash is included in server URL. Contributed by @angdraug.
diff --git a/synapse/_scripts/register_new_matrix_user.py b/synapse/_scripts/register_new_matrix_user.py
index d37ccccd5b..dfe26dea6d 100644
--- a/synapse/_scripts/register_new_matrix_user.py
+++ b/synapse/_scripts/register_new_matrix_user.py
@@ -37,7 +37,7 @@ def request_registration(
     exit=sys.exit,
 ):
 
-    url = "%s/_synapse/admin/v1/register" % (server_location,)
+    url = "%s/_synapse/admin/v1/register" % (server_location.rstrip("/"),)
 
     # Get the nonce
     r = requests.get(url, verify=False)