diff --git a/lisp/url/url-util.el b/lisp/url/url-util.el
index 8435114bbc2..47ef8811c5e 100644
--- a/lisp/url/url-util.el
+++ b/lisp/url/url-util.el
@@ -409,6 +409,13 @@ should return it unchanged."
 	      (url-hexify-string frag url-query-allowed-chars)))
     (url-recreate-url obj)))
 
+(defconst url--query-key-value-preserved-chars
+  (let ((vec (copy-sequence url-query-key-value-allowed-chars)))
+    (aset vec ?% nil)
+    vec)
+  "Byte mask used for %-encoding keys and values in the query segment of a URI.
+`url-query-key-value-allowed-chars' minus '%'.")
+
 ;;;###autoload
 (defun url-build-query-string (query &optional semicolons keep-empty)
   "Build a query-string.
@@ -436,7 +443,7 @@ instead of just \"key\" as in the example above."
      (let ((escaped
             (mapcar (lambda (sym)
                       (url-hexify-string (format "%s" sym)
-                                         url-query-key-value-allowed-chars))
+                                         url--query-key-value-preserved-chars))
                     key-vals)))
        (mapconcat (lambda (val)
                     (let ((vprint (format "%s" val))
diff --git a/test/lisp/url/url-util-tests.el b/test/lisp/url/url-util-tests.el
index d34bfc8509c..6e79a798333 100644
--- a/test/lisp/url/url-util-tests.el
+++ b/test/lisp/url/url-util-tests.el
@@ -33,10 +33,10 @@
             ((key1 "val1") (key2 val2) (key3 val1 val2) ("key4") (key5 "")) t)
            ("key1=val1;key2=val2;key3=val1;key3=val2;key4=;key5="
             ((key1 val1) (key2 val2) ("key3" val1 val2) (key4) (key5 "")) t t)
-           ("key1=val/slash;key2=val%3Bsemi;key3=val%26amp;key4=val%3Deq"
-            ((key1 "val/slash") (key2 "val;semi") (key3 "val&amp") (key4 "val=eq")) t)
-           ("key%3Deq=val1;key%3Bsemi=val2;key%26amp=val3"
-            (("key=eq" val1) ("key;semi" val2) ("key&amp" val3)) t)))
+           ("key1=val/slash;key2=val%3Bsemi;key3=val%26amp;key4=val%3Deq;key5=val%25perc"
+            ((key1 "val/slash") (key2 "val;semi") (key3 "val&amp") (key4 "val=eq") (key5 "val%perc")) t)
+           ("key%3Deq=val1;key%3Bsemi=val2;key%26amp=val3;key%25perc=val4"
+            (("key=eq" val1) ("key;semi" val2) ("key&amp" val3) ("key%perc" val4)) t)))
         test)
     (while tests
       (setq test (car tests)