diff --git a/iron-validatable-behavior.html b/iron-validatable-behavior.html
index 803731b..0be57aa 100644
--- a/iron-validatable-behavior.html
+++ b/iron-validatable-behavior.html
@@ -46,10 +46,11 @@
     properties: {
 
       /**
-       * Name of the validator to use.
+       * Name of the validator to use, or the validating function.
+       * @type {string|function(Object):boolean}
        */
       validator: {
-        type: String
+        type: Object
       },
 
       /**
@@ -140,9 +141,20 @@
       return true;
     },
 
-    __computeValidator: function() {
-      return Polymer.IronValidatableBehaviorMeta &&
-          Polymer.IronValidatableBehaviorMeta.byKey(this.validator);
+    /**
+     * Retrieves the validator.
+     *
+     * @param {string|function(Object):boolean} validator
+     *   The validator name or function.
+     * @return {!{validate: function(Object):boolean}} The validator.
+     */
+    __computeValidator: function(validator) {
+      if (typeof validator === 'function') {
+        return {validate: validator};
+      } else {
+        return Polymer.IronValidatableBehaviorMeta &&
+            Polymer.IronValidatableBehaviorMeta.byKey(validator);
+      }
     }
   };
 
diff --git a/test/iron-validatable-behavior.html b/test/iron-validatable-behavior.html
index a8040ef..96df9b2 100644
--- a/test/iron-validatable-behavior.html
+++ b/test/iron-validatable-behavior.html
@@ -74,13 +74,17 @@
         input.validator = 'cats-only';
         assert.isFalse(input.validate('ca'));
         assert.isTrue(input.validate('cats'));
+        
+        // Only valid if the value is 'lizards'.
+        input.validator = (value) => value === 'lizards';
+        assert.isFalse(input.validate('cats'));
+        assert.isTrue(input.validate('lizards'));
 
         // Only valid if the value is 'dogs'.
         input.validator = 'dogs-only';
         assert.isFalse(input.validate('cats'));
         assert.isTrue(input.validate('dogs'));
       });
-
     });
 
   </script>