diff --git a/docs/guide/input-validation.md b/docs/guide/input-validation.md
index 8acfb6e322..532945d211 100644
--- a/docs/guide/input-validation.md
+++ b/docs/guide/input-validation.md
@@ -487,6 +487,7 @@ predefined variables:
 - `attribute`: the name of the attribute being validated.
 - `value`: the value being validated.
 - `messages`: an array used to hold the validation error messages for the attribute.
+- `deferred`: an array which deferred objects can be pushed into.
 
 In the following example, we create a `StatusValidator` which validates if an input is a valid status input
 against the existing status data. The validator supports both server side and client side validation.
@@ -535,6 +536,57 @@ JS;
 ]
 ```
 
+### Deferred validation
+
+If you need to perform any asynchronous validation you can use a [deferred object](http://api.jquery.com/category/deferred-object/).
+
+deferred objects must be pushed to the ```deferred``` array for validation to use them.
+Once any asynchronous validation has finished you must call ```resolve()``` on the Deferred object for it to complete.
+
+This example shows reading an image to check the dimensions client side (```file``` will be from an input of type=file).
+```php
+...
+public function clientValidateAttribute($model, $attribute, $view)
+{
+    return <<<JS
+    var def = $.Deferred();
+    var img = new Image();
+    img.onload = function() {
+        if (this.width > 150) {
+            messages.push('Image too wide!!');
+        }
+        def.resolve();
+    }
+    var reader = new FileReader();
+    reader.onloadend = function() {
+        img.src = reader.result;
+    }
+    reader.readAsDataURL(file);
+
+    deferred.push(def);
+JS;
+}
+...
+```
+
+Ajax can also be used and pushed straight into the deferred array.
+```
+deferred.push($.get("/check", {value: value}).done(function(data) {
+    if ('' !== data) {
+        messages.push(data);
+    }
+}));
+```
+
+The ```deferred``` array also has a shortcut method ```add```.
+```
+deferred.add(function(def) {
+    //Asynchronous Validation here
+    //The context of this function and the first argument is the Deferred object where resolve can be called.
+});
+```
+>   Note: `resolve` must be called on any deferred objects after the attribute has been validated or the main form validation will not complete.
+
 ### Ajax validation
 
 Some kind of validation can only be done on server side because only the server has the necessary information
diff --git a/framework/CHANGELOG.md b/framework/CHANGELOG.md
index 23138182cf..e0e8a35891 100644
--- a/framework/CHANGELOG.md
+++ b/framework/CHANGELOG.md
@@ -165,6 +165,7 @@ Yii Framework 2 Change Log
 - Enh #4317: Added `absoluteAuthTimeout` to yii\web\User (ivokund, nkovacs)
 - Enh #4360: Added client validation support for file validator (Skysplit)
 - Enh #4436: Added callback functions to AJAX-based form validation (thiagotalma)
+- Enh #4485: Added support for deferred validation in `ActiveForm` (Alex-Code)
 - Enh #4520: Added sasl support to `yii\caching\MemCache` (xjflyttp)
 - Enh: Added support for using sub-queries when building a DB query with `IN` condition (qiangxue)
 - Enh: Supported adding a new response formatter without the need to reconfigure existing formatters (qiangxue)
diff --git a/framework/assets/yii.activeForm.js b/framework/assets/yii.activeForm.js
index 9f8c6159d3..bab3fb5936 100644
--- a/framework/assets/yii.activeForm.js
+++ b/framework/assets/yii.activeForm.js
@@ -270,7 +270,20 @@
             });
         }, data.settings.validationDelay);
     };
-
+    
+    /**
+     * Returns an array prototype with a shortcut method for adding a new deferred.
+     * The context of the callback will be the deferred object so it can be resolved like ```this.resolve()```
+     * @returns Array
+     */
+    var deferredArray = function () {
+        var array = [];
+        array.add = function(callback) {
+            this.push(new $.Deferred(callback));
+        };
+        return array;
+    };
+    
     /**
      * Performs validation.
      * @param $form jQuery the jquery representation of the form
@@ -280,70 +293,78 @@
     var validate = function ($form, successCallback, errorCallback) {
         var data = $form.data('yiiActiveForm'),
             needAjaxValidation = false,
-            messages = {};
+            messages = {},
+            deferreds = deferredArray();
 
         $.each(data.attributes, function () {
             if (data.submitting || this.status === 2 || this.status === 3) {
                 var msg = [];
+                messages[this.id] = msg;
                 if (!data.settings.beforeValidate || data.settings.beforeValidate($form, this, msg)) {
                     if (this.validate) {
-                        this.validate(this, getValue($form, this), msg);
+                        this.validate(this, getValue($form, this), msg, deferreds);
                     }
-                    if (msg.length) {
-                        messages[this.id] = msg;
-                    } else if (this.enableAjaxValidation) {
+                    if (this.enableAjaxValidation) {
                         needAjaxValidation = true;
                     }
                 }
             }
         });
 
-        if (needAjaxValidation && (!data.submitting || $.isEmptyObject(messages))) {
-            // Perform ajax validation when at least one input needs it.
-            // If the validation is triggered by form submission, ajax validation
-            // should be done only when all inputs pass client validation
-            var $button = data.submitObject,
-                extData = '&' + data.settings.ajaxParam + '=' + $form.prop('id');
-            if ($button && $button.length && $button.prop('name')) {
-                extData += '&' + $button.prop('name') + '=' + $button.prop('value');
+        $.when.apply(this, deferreds).always(function() {
+            //Remove empty message arrays
+            for (var i in messages) {
+                if (0 === messages[i].length) {
+                    delete messages[i];
+                }
             }
-            $.ajax({
-                url: data.settings.validationUrl,
-                type: $form.prop('method'),
-                data: $form.serialize() + extData,
-                dataType: data.settings.ajaxDataType,
-                complete: function (jqXHR, textStatus) {
-                    if (data.settings.ajaxComplete) {
-                        data.settings.ajaxComplete($form, jqXHR, textStatus);
-                    }
-                },
-                beforeSend: function (jqXHR, textStatus) {
-                    if (data.settings.ajaxBeforeSend) {
-                        data.settings.ajaxBeforeSend($form, jqXHR, textStatus);
-                    }
-                },
-                success: function (msgs) {
-                    if (msgs !== null && typeof msgs === 'object') {
-                        $.each(data.attributes, function () {
-                            if (!this.enableAjaxValidation) {
-                                delete msgs[this.id];
-                            }
-                        });
-                        successCallback($.extend({}, messages, msgs));
-                    } else {
-                        successCallback(messages);
-                    }
-                },
-                error: errorCallback
-            });
-        } else if (data.submitting) {
-            // delay callback so that the form can be submitted without problem
-            setTimeout(function () {
+            if (needAjaxValidation && (!data.submitting || $.isEmptyObject(messages))) {
+                // Perform ajax validation when at least one input needs it.
+                // If the validation is triggered by form submission, ajax validation
+                // should be done only when all inputs pass client validation
+                var $button = data.submitObject,
+                    extData = '&' + data.settings.ajaxParam + '=' + $form.prop('id');
+                if ($button && $button.length && $button.prop('name')) {
+                    extData += '&' + $button.prop('name') + '=' + $button.prop('value');
+                }
+                $.ajax({
+                    url: data.settings.validationUrl,
+                    type: $form.prop('method'),
+                    data: $form.serialize() + extData,
+                    dataType: data.settings.ajaxDataType,
+                    complete: function (jqXHR, textStatus) {
+                        if (data.settings.ajaxComplete) {
+                            data.settings.ajaxComplete($form, jqXHR, textStatus);
+                        }
+                    },
+                    beforeSend: function (jqXHR, textStatus) {
+                        if (data.settings.ajaxBeforeSend) {
+                            data.settings.ajaxBeforeSend($form, jqXHR, textStatus);
+                        }
+                    },
+                    success: function (msgs) {
+                        if (msgs !== null && typeof msgs === 'object') {
+                            $.each(data.attributes, function () {
+                                if (!this.enableAjaxValidation) {
+                                    delete msgs[this.id];
+                                }
+                            });
+                            successCallback($.extend({}, messages, msgs));
+                        } else {
+                            successCallback(messages);
+                        }
+                    },
+                    error: errorCallback
+                });
+            } else if (data.submitting) {
+                // delay callback so that the form can be submitted without problem
+                setTimeout(function () {
+                    successCallback(messages);
+                }, 200);
+            } else {
                 successCallback(messages);
-            }, 200);
-        } else {
-            successCallback(messages);
-        }
+            }
+        });
     };
 
     /**
diff --git a/framework/widgets/ActiveField.php b/framework/widgets/ActiveField.php
index 956e0d9ce8..636db97fe5 100644
--- a/framework/widgets/ActiveField.php
+++ b/framework/widgets/ActiveField.php
@@ -703,7 +703,7 @@ class ActiveField extends Component
                 }
             }
             if (!empty($validators)) {
-                $options['validate'] = new JsExpression("function (attribute, value, messages) {" . implode('', $validators) . '}');
+                $options['validate'] = new JsExpression("function (attribute, value, messages, deferred) {" . implode('', $validators) . '}');
             }
         }