diff --git a/changelog.d/4555.bugfix b/changelog.d/4555.bugfix
new file mode 100644
index 000000000000..d596022c3fda
--- /dev/null
+++ b/changelog.d/4555.bugfix
@@ -0,0 +1 @@
+Avoid redundant URL encoding of redirect URL for SSO login in the fallback login page. Fixes a regression introduced in [#4220](https://github.com/matrix-org/synapse/pull/4220). Contributed by Marcel Fabian Krüger ("[zaugin](https://github.com/zauguin)").
diff --git a/synapse/static/client/login/js/login.js b/synapse/static/client/login/js/login.js
index 3a958749a19a..e02663f50e9d 100644
--- a/synapse/static/client/login/js/login.js
+++ b/synapse/static/client/login/js/login.js
@@ -49,7 +49,7 @@ var show_login = function() {
     $("#loading").hide();
 
     var this_page = window.location.origin + window.location.pathname;
-    $("#sso_redirect_url").val(encodeURIComponent(this_page));
+    $("#sso_redirect_url").val(this_page);
 
     if (matrixLogin.serverAcceptsPassword) {
         $("#password_flow").show();