diff --git a/changelog.d/6063.bugfix b/changelog.d/6063.bugfix
new file mode 100644
index 000000000..7485e32a2
--- /dev/null
+++ b/changelog.d/6063.bugfix
@@ -0,0 +1 @@
+Ensure query parameters in email validation links are URL-encoded.
\ No newline at end of file
diff --git a/synapse/push/mailer.py b/synapse/push/mailer.py
index 3dfd52784..2437235dc 100644
--- a/synapse/push/mailer.py
+++ b/synapse/push/mailer.py
@@ -136,10 +136,11 @@ class Mailer(object):
                 group together multiple email sending attempts
             sid (str): The generated session ID
         """
+        params = {"token": token, "client_secret": client_secret, "sid": sid}
         link = (
             self.hs.config.public_baseurl
-            + "_matrix/client/unstable/password_reset/email/submit_token"
-            "?token=%s&client_secret=%s&sid=%s" % (token, client_secret, sid)
+            + "_matrix/client/unstable/password_reset/email/submit_token?%s"
+            % urllib.parse.urlencode(params)
         )
 
         template_vars = {"link": link}
@@ -163,10 +164,11 @@ class Mailer(object):
                 group together multiple email sending attempts
             sid (str): The generated session ID
         """
+        params = {"token": token, "client_secret": client_secret, "sid": sid}
         link = (
             self.hs.config.public_baseurl
-            + "_matrix/client/unstable/registration/email/submit_token"
-            "?token=%s&client_secret=%s&sid=%s" % (token, client_secret, sid)
+            + "_matrix/client/unstable/registration/email/submit_token?%s"
+            % urllib.parse.urlencode(params)
         )
 
         template_vars = {"link": link}