diff --git a/framework/CHANGELOG.md b/framework/CHANGELOG.md
index 44b8a3d..0d641d5 100644
--- a/framework/CHANGELOG.md
+++ b/framework/CHANGELOG.md
@@ -183,9 +183,10 @@ Yii Framework 2 Change Log
 - Enh #4607: AR model will throw an exception if it does not have a primary key to avoid updating/deleting data massively (qiangxue)
 - Enh #4630: Added automatic generating of unique slug value to `yii\behaviors\Sluggable` (klimov-paul)
 - Enh #4636: Added `yii\web\Response::setDownloadHeaders()` (pawzar)
-- Enh #4644: Added `\yii\db\Schema::createColumnSchema()` to be able to customize column schema used (mcd-php)
+- Enh #4644: Added `yii\db\Schema::createColumnSchema()` to be able to customize column schema used (mcd-php)
 - Enh #4656: HtmlPurifier helper config can now be a closure to change the purifier config object after it was created (Alex-Code)
 - Enh #4691: Encoding on `ActiveForm` and `ActiveField` validation errors is now configurable (Alex-Code)
+- Enh #4740: Added `yii\web\Session::addFlash()` (restyler)
 - 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)
 - Enh: Added `yii\web\UrlManager::addRules()` to simplify adding new URL rules (qiangxue)
diff --git a/framework/web/Session.php b/framework/web/Session.php
index f9a80cc..76dac90 100644
--- a/framework/web/Session.php
+++ b/framework/web/Session.php
@@ -699,9 +699,10 @@ class Session extends Component implements \IteratorAggregate, \ArrayAccess, \Co
     }
 
     /**
-     * Stores a flash message.
+     * Sets a flash message.
      * A flash message will be automatically deleted after it is accessed in a request and the deletion will happen
      * in the next request.
+     * If there is already an existing flash message with the same key, it will be overwritten by the new one.
      * @param string $key the key identifying the flash message. Note that flash messages
      * and normal session variables share the same name space. If you have a normal
      * session variable using the same name, its value will be overwritten by this method.
@@ -722,14 +723,14 @@ class Session extends Component implements \IteratorAggregate, \ArrayAccess, \Co
     }
 
     /**
-     * Appends new flash message to the specified key.
+     * Adds a flash message.
+     * If there are existing flash messages with the same key, the new one will be appended to the existing message array.
      * @param string $key the key identifying the flash message.
      * @param mixed $value flash message
      * @param boolean $removeAfterAccess whether the flash message should be automatically removed only if
      * it is accessed. If false, the flash message will be automatically removed after the next request,
      * regardless if it is accessed or not. If true (default value), the flash message will remain until after
      * it is accessed.
-     * @throws InvalidParamException if exising session variable is not array.
      * @see getFlash()
      * @see removeFlash()
      */
@@ -737,16 +738,16 @@ class Session extends Component implements \IteratorAggregate, \ArrayAccess, \Co
     {
         $counters = $this->get($this->flashParam, []);
         $counters[$key] = $removeAfterAccess ? -1 : 0;
-        if (!empty($_SESSION[$key])) {
-            // If it's not an array, convert it to array 
-            if (!is_array($_SESSION[$key])) {
-                $_SESSION[$key] = [$_SESSION[$key]];
-            }
-            $_SESSION[$key][] = $value;
-        } else {
+        $_SESSION[$this->flashParam] = $counters;
+        if (empty($_SESSION[$key])) {
             $_SESSION[$key] = [$value];
+        } else {
+            if (is_array($_SESSION[$key])) {
+                $_SESSION[$key][] = $value;
+            } else {
+                $_SESSION[$key] = [$_SESSION[$key], $value];
+            }
         }
-        $_SESSION[$this->flashParam] = $counters;
     }
 
     /**