diff --git a/framework/CHANGELOG.md b/framework/CHANGELOG.md
index 14cff76..7dd79f4 100644
--- a/framework/CHANGELOG.md
+++ b/framework/CHANGELOG.md
@@ -16,6 +16,7 @@ Yii Framework 2 Change Log
 - Enh #12938: Allow to pass additional parameters to `yii\base\View::renderDynamic()` (mikehaertl)
 - Enh #13006: Added a `/` to the `yii\captcha\Captcha::$captchaAction` string to work correctly in a module also (boehsermoe)
 - Removed methods marked as deprecated in 2.0.x (samdark)
+- Chg #14784: Signature of `yii\web\RequestParserInterface::parse()` changed to accept `yii\web\Request` instance as a sole argument (klimov-paul)
 - Chg #10771: Consistent behavior of `run()` method in all framework widgets. All return the result now for better extensibility (pkirill99, cebe)
 - Chg #11397: Minimum required version of PHP is 7.1 now (samdark)
 - Chg: Removed `yii\base\Object::className()` in favor of native PHP syntax `::class`, which does not trigger autoloading (cebe)
diff --git a/framework/UPGRADE.md b/framework/UPGRADE.md
index f4c57d5..89800e5 100644
--- a/framework/UPGRADE.md
+++ b/framework/UPGRADE.md
@@ -110,6 +110,8 @@ Upgrade from Yii 2.0.x
   with PHP. For details please refer to [guide on autoloading](https://github.com/yiisoft/yii2/blob/2.1/docs/guide/concept-autoloading.md),  
   [guide on customizing helpers](https://github.com/yiisoft/yii2/blob/2.1/docs/guide/helper-overview.md#customizing-helper-classes-)
   and [guide on Working with Third-Party Code](https://github.com/yiisoft/yii2/blob/2.1/docs/guide/tutorial-yii-integration.md).
+* The signature of `yii\web\RequestParserInterface::parse()` was changed. The method now accepts the `yii\web\Request` instance
+  as a sole argument. Make sure you declare and implement this method correctly, while creating your own request parser.
 
 
 Upgrade from Yii 2.0.12
diff --git a/framework/web/JsonParser.php b/framework/web/JsonParser.php
index f9e6964..4398714 100644
--- a/framework/web/JsonParser.php
+++ b/framework/web/JsonParser.php
@@ -39,16 +39,13 @@ class JsonParser implements RequestParserInterface
 
 
     /**
-     * Parses a HTTP request body.
-     * @param string $rawBody the raw HTTP request body.
-     * @param string $contentType the content type specified for the request body.
-     * @return array parameters parsed from the request body
+     * {@inheritdoc}
      * @throws BadRequestHttpException if the body contains invalid json and [[throwException]] is `true`.
      */
-    public function parse($rawBody, $contentType)
+    public function parse($request)
     {
         try {
-            $parameters = Json::decode($rawBody, $this->asArray);
+            $parameters = Json::decode($request->getBody()->__toString(), $this->asArray);
             return $parameters === null ? [] : $parameters;
         } catch (InvalidArgumentException $e) {
             if ($this->throwException) {
diff --git a/framework/web/MultipartFormDataParser.php b/framework/web/MultipartFormDataParser.php
index 0ca70ab..54e1997 100644
--- a/framework/web/MultipartFormDataParser.php
+++ b/framework/web/MultipartFormDataParser.php
@@ -123,9 +123,9 @@ class MultipartFormDataParser extends BaseObject implements RequestParserInterfa
     }
 
     /**
-     * @inheritdoc
+     * {@inheritdoc}
      */
-    public function parse($rawBody, $contentType)
+    public function parse($request)
     {
         if (!$this->force) {
             if (!empty($_POST) || !empty($_FILES)) {
@@ -136,6 +136,9 @@ class MultipartFormDataParser extends BaseObject implements RequestParserInterfa
             $_FILES = [];
         }
 
+        $contentType = $request->getContentType();
+        $rawBody = $request->getBody()->__toString();
+
         if (empty($rawBody)) {
             return [];
         }
diff --git a/framework/web/Request.php b/framework/web/Request.php
index e4a193b..a2ea991 100644
--- a/framework/web/Request.php
+++ b/framework/web/Request.php
@@ -16,6 +16,7 @@ use yii\di\Instance;
 use yii\http\Cookie;
 use yii\http\CookieCollection;
 use yii\http\FileStream;
+use yii\http\MemoryStream;
 use yii\http\MessageTrait;
 use yii\http\Uri;
 
@@ -479,19 +480,13 @@ class Request extends \yii\base\Request implements RequestInterface
         ]);
     }
 
-    private $_rawBody;
-
     /**
      * Returns the raw HTTP request body.
      * @return string the request body
      */
     public function getRawBody()
     {
-        if ($this->_rawBody === null) {
-            $this->_rawBody = $this->getBody()->__toString();
-        }
-
-        return $this->_rawBody;
+        return $this->getBody()->__toString();
     }
 
     /**
@@ -500,7 +495,9 @@ class Request extends \yii\base\Request implements RequestInterface
      */
     public function setRawBody($rawBody)
     {
-        $this->_rawBody = $rawBody;
+        $body = new MemoryStream();
+        $body->write($rawBody);
+        $this->setBody($body);
     }
 
     private $_bodyParams;
@@ -539,13 +536,13 @@ class Request extends \yii\base\Request implements RequestInterface
                 if (!($parser instanceof RequestParserInterface)) {
                     throw new InvalidConfigException("The '$contentType' request parser is invalid. It must implement the yii\\web\\RequestParserInterface.");
                 }
-                $this->_bodyParams = $parser->parse($this->getRawBody(), $rawContentType);
+                $this->_bodyParams = $parser->parse($this);
             } elseif (isset($this->parsers['*'])) {
                 $parser = Yii::createObject($this->parsers['*']);
                 if (!($parser instanceof RequestParserInterface)) {
                     throw new InvalidConfigException('The fallback request parser is invalid. It must implement the yii\\web\\RequestParserInterface.');
                 }
-                $this->_bodyParams = $parser->parse($this->getRawBody(), $rawContentType);
+                $this->_bodyParams = $parser->parse($this);
             } elseif ($this->getMethod() === 'POST') {
                 // PHP has already parsed the body so we have all params in $_POST
                 $this->_bodyParams = $_POST;
diff --git a/framework/web/RequestParserInterface.php b/framework/web/RequestParserInterface.php
index 2108b82..4b58153 100644
--- a/framework/web/RequestParserInterface.php
+++ b/framework/web/RequestParserInterface.php
@@ -11,15 +11,18 @@ namespace yii\web;
  * Interface for classes that parse the raw request body into a parameters array.
  *
  * @author Dan Schmidt <danschmidt5189@gmail.com>
+ * @author Paul Klimov <klimov.paul@gmail.com>
  * @since 2.0
  */
 interface RequestParserInterface
 {
     /**
-     * Parses a HTTP request body.
-     * @param string $rawBody the raw HTTP request body.
-     * @param string $contentType the content type specified for the request body.
-     * @return array parameters parsed from the request body
+     * Parses a given request instance determining its body parameters.
+     * This method MUST return the array of body parameters detected from [[$request]] data.
+     * However, this method MAY adjust the given [[Request]] instance directly for extra configuration.
+     * @param Request $request the HTTP request instance to be parsed.
+     * @return array parameters parsed from the request body.
+     * @throws BadRequestHttpException in case request body format is invalid.
      */
-    public function parse($rawBody, $contentType);
+    public function parse($request);
 }
diff --git a/tests/framework/web/MultipartFormDataParserTest.php b/tests/framework/web/MultipartFormDataParserTest.php
index c4b2f82..2dabf50 100644
--- a/tests/framework/web/MultipartFormDataParserTest.php
+++ b/tests/framework/web/MultipartFormDataParserTest.php
@@ -8,6 +8,7 @@
 namespace yiiunit\framework\web;
 
 use yii\web\MultipartFormDataParser;
+use yii\web\Request;
 use yiiunit\TestCase;
 
 class MultipartFormDataParserTest extends TestCase
@@ -24,7 +25,14 @@ class MultipartFormDataParserTest extends TestCase
         $rawBody .= "\r\n--{$boundary}\nContent-Disposition: form-data; name=\"Item[file]\"; filename=\"item-file.txt\"\nContent-Type: text/plain\r\n\r\nitem file content";
         $rawBody .= "\r\n--{$boundary}--";
 
-        $bodyParams = $parser->parse($rawBody, $contentType);
+        $request = new Request([
+            'rawBody' => $rawBody,
+            'headers' => [
+                'content-type' => [$contentType]
+            ]
+        ]);
+
+        $bodyParams = $parser->parse($request);
 
         $expectedBodyParams = [
             'title' => 'test-title',
@@ -59,7 +67,13 @@ class MultipartFormDataParserTest extends TestCase
             'name' => 'value',
         ];
 
-        $bodyParams = $parser->parse('should not matter', 'multipart/form-data; boundary=---12345');
+        $request = new Request([
+            'rawBody' => 'should not matter',
+            'headers' => [
+                'content-type' => ['multipart/form-data; boundary=---12345']
+            ]
+        ]);
+        $bodyParams = $parser->parse($request);
         $this->assertEquals($_POST, $bodyParams);
         $this->assertEquals([], $_FILES);
     }
@@ -82,7 +96,13 @@ class MultipartFormDataParserTest extends TestCase
         $contentType = 'multipart/form-data; boundary=' . $boundary;
         $rawBody = "--{$boundary}\nContent-Disposition: form-data; name=\"title\"\r\ntest-title--{$boundary}--";
 
-        $bodyParams = $parser->parse($rawBody, $contentType);
+        $request = new Request([
+            'rawBody' => $rawBody,
+            'headers' => [
+                'content-type' => [$contentType]
+            ]
+        ]);
+        $bodyParams = $parser->parse($request);
         $this->assertEquals([], $bodyParams);
     }
 
@@ -101,7 +121,13 @@ class MultipartFormDataParserTest extends TestCase
         $rawBody .= "--{$boundary}\nContent-Disposition: form-data; name=\"thirdFile\"; filename=\"third-file.txt\"\nContent-Type: text/plain\r\n\r\nthird file content";
         $rawBody .= "--{$boundary}--";
 
-        $parser->parse($rawBody, $contentType);
+        $request = new Request([
+            'rawBody' => $rawBody,
+            'headers' => [
+                'content-type' => [$contentType]
+            ]
+        ]);
+        $parser->parse($request);
         $this->assertCount(2, $_FILES);
     }
 
@@ -120,7 +146,13 @@ class MultipartFormDataParserTest extends TestCase
         $rawBody .= "--{$boundary}\nContent-Disposition: form-data; name=\"thirdFile\"; filename=\"third-file.txt\"\nContent-Type: text/plain\r\n\r\nthird file with too long file content";
         $rawBody .= "--{$boundary}--";
 
-        $parser->parse($rawBody, $contentType);
+        $request = new Request([
+            'rawBody' => $rawBody,
+            'headers' => [
+                'content-type' => [$contentType]
+            ]
+        ]);
+        $parser->parse($request);
         $this->assertCount(3, $_FILES);
         $this->assertEquals(UPLOAD_ERR_INI_SIZE, $_FILES['thirdFile']['error']);
     }
@@ -150,7 +182,13 @@ class MultipartFormDataParserTest extends TestCase
         $rawBody .= "\r\n--{$boundary}\nContent-Disposition: form-data; name=\"someFile\"; filename=\"some-file.txt\"\nContent-Type: text/plain\r\n\r\nsome file content";
         $rawBody .= "\r\n--{$boundary}--";
 
-        $bodyParams = $parser->parse($rawBody, $contentType);
+        $request = new Request([
+            'rawBody' => $rawBody,
+            'headers' => [
+                'content-type' => [$contentType]
+            ]
+        ]);
+        $bodyParams = $parser->parse($request);
 
         $expectedBodyParams = [
             'title' => 'test-title',