diff --git a/Lib/decimal.py b/Lib/decimal.py
--- a/Lib/decimal.py
+++ b/Lib/decimal.py
@@ -3759,6 +3759,8 @@
         if self._is_special:
             sign = _format_sign(self._sign, spec)
             body = str(self.copy_abs())
+            if spec['type'] == '%':
+                body += '%'
             return _format_align(sign, body, spec)
 
         # a type of None defaults to 'g' or 'G', depending on context
diff --git a/Lib/test/test_decimal.py b/Lib/test/test_decimal.py
--- a/Lib/test/test_decimal.py
+++ b/Lib/test/test_decimal.py
@@ -1058,6 +1058,11 @@
 
             # issue 6850
             ('a=-7.0', '0.12345', 'aaaa0.1'),
+
+            # issue 22090
+            ('<^+15.20%', 'inf', '<<+Infinity%<<<'),
+            ('\x07>,%', 'sNaN1234567', 'sNaN1234567%'),
+            ('=10.10%', 'NaN123', '   NaN123%'),
             ]
         for fmt, d, result in test_values:
             self.assertEqual(format(Decimal(d), fmt), result)
diff --git a/Modules/_decimal/libmpdec/io.c b/Modules/_decimal/libmpdec/io.c
--- a/Modules/_decimal/libmpdec/io.c
+++ b/Modules/_decimal/libmpdec/io.c
@@ -446,7 +446,7 @@
 
     if (mpd_isspecial(dec)) {
 
-        mem = sizeof "-Infinity";
+        mem = sizeof "-Infinity%";
         if (mpd_isnan(dec) && dec->len > 0) {
             /* diagnostic code */
             mem += dec->digits;
@@ -609,10 +609,10 @@
             *cp++ = (flags&MPD_FMT_UPPER) ? 'E' : 'e';
             cp = exp_to_string(cp, ldigits-dplace);
         }
+    }
 
-        if (flags&MPD_FMT_PERCENT) {
-            *cp++ = '%';
-        }
+    if (flags&MPD_FMT_PERCENT) {
+        *cp++ = '%';
     }
 
     assert(cp < decstring+mem);
@@ -1260,6 +1260,9 @@
             stackspec.align = '>';
             spec = &stackspec;
         }
+        if (type == '%') {
+            flags |= MPD_FMT_PERCENT;
+        }
     }
     else {
         uint32_t workstatus = 0;