diff --git a/validator/testdata/css/media-not-prefers-reduced-motion.css b/validator/testdata/css/media-not-prefers-reduced-motion.css
new file mode 100644
index 000000000000..221ff782bd91
--- /dev/null
+++ b/validator/testdata/css/media-not-prefers-reduced-motion.css
@@ -0,0 +1,5 @@
+@media not (prefers-reduced-motion: reduce) {
+  .test {
+    visibility: hidden;
+  }
+}
diff --git a/validator/testdata/feature_tests/everything.html b/validator/testdata/feature_tests/everything.html
index b5c00917cba5..6049767f81f9 100644
--- a/validator/testdata/feature_tests/everything.html
+++ b/validator/testdata/feature_tests/everything.html
@@ -112,6 +112,13 @@
       background: rgba(0, 0, 0, 0.5);
     }
 
+    /* Test: negated prefers-reduced-motion media query */
+    @media not all and (prefers-reduced-motion: reduce) {
+      body {
+        background: red;
+      }
+    }
+
 
   </style>
   <script async custom-element="amp-dynamic-css-classes" src="https://cdn.ampproject.org/v0/amp-dynamic-css-classes-0.1.js"></script>
diff --git a/validator/testdata/feature_tests/everything.out b/validator/testdata/feature_tests/everything.out
index 92179e5fd0a6..73f4e65ec991 100644
--- a/validator/testdata/feature_tests/everything.out
+++ b/validator/testdata/feature_tests/everything.out
@@ -113,6 +113,13 @@ PASS
 |        background: rgba(0, 0, 0, 0.5);
 |      }
 |
+|      /* Test: negated prefers-reduced-motion media query */
+|      @media not all and (prefers-reduced-motion: reduce) {
+|        body {
+|          background: red;
+|        }
+|      }
+|
 |
 |    </style>
 |    <script async custom-element="amp-dynamic-css-classes" src="https://cdn.ampproject.org/v0/amp-dynamic-css-classes-0.1.js"></script>
@@ -451,4 +458,4 @@ PASS
 |    </div>
 |  </article>
 |  </body>
-|  </html>
+|  </html>
\ No newline at end of file
diff --git a/validator/validator-css.protoascii b/validator/validator-css.protoascii
index a73c09538e6b..d1b969ce55f1 100644
--- a/validator/validator-css.protoascii
+++ b/validator/validator-css.protoascii
@@ -1229,6 +1229,7 @@ tags: {  # `<style amp-keyframes>`, [AMP, AMP4ADS]
            type: 'braille'
            type: 'embossesd'
            type: 'aural'
+           type: 'not'
            feature: 'any-hover'
            feature: 'any-pointer'
            feature: 'aspect-ratio'