diff --git a/build/phpstan/joomla-bootstrap.php b/build/phpstan/joomla-bootstrap.php
index c285e232b6c25..21178069ab352 100644
--- a/build/phpstan/joomla-bootstrap.php
+++ b/build/phpstan/joomla-bootstrap.php
@@ -10,7 +10,6 @@
  */
 
 \define('_JEXEC', 1);
-\define('JPATH_PLATFORM', 1);
 \define('JPATH_BASE', \dirname(__DIR__, 2));
 
 // Load the Joomla environment
diff --git a/build/psr12/clean_errors.php b/build/psr12/clean_errors.php
index 257cd183d49f8..3db025ea466fd 100644
--- a/build/psr12/clean_errors.php
+++ b/build/psr12/clean_errors.php
@@ -40,10 +40,6 @@
                                     "defined('_JEXEC') or die();",
                                     "\defined('_JEXEC') or die;",
                                     "defined('_JEXEC') or die;",
-                                    "\defined('JPATH_PLATFORM') or die();",
-                                    "defined('JPATH_PLATFORM') or die();",
-                                    "\defined('JPATH_PLATFORM') or die;",
-                                    "defined('JPATH_PLATFORM') or die;",
                                     "\defined('JPATH_BASE') or die();",
                                     "defined('JPATH_BASE') or die();",
                                     "\defined('JPATH_BASE') or die;",
diff --git a/libraries/bootstrap.php b/libraries/bootstrap.php
index 190240a730ee6..713e69ce985f9 100644
--- a/libraries/bootstrap.php
+++ b/libraries/bootstrap.php
@@ -10,14 +10,6 @@
 
 defined('_JEXEC') or die;
 
-/**
- * Set the platform root path as a constant if necessary.
- *
- * @deprecated 4.4.0 will be removed in 6.0
- *             Use defined('_JEXEC') or die; to detect if the CMS is loaded correctly
- **/
-defined('JPATH_PLATFORM') or define('JPATH_PLATFORM', __DIR__);
-
 // Detect the native operating system type.
 $os = strtoupper(substr(PHP_OS, 0, 3));
 
diff --git a/libraries/cms.php b/libraries/cms.php
index 03f8203174b7b..b117d028f755d 100644
--- a/libraries/cms.php
+++ b/libraries/cms.php
@@ -18,16 +18,6 @@
     E_USER_DEPRECATED
 );
 
-/**
- * Set the platform root path as a constant if necessary.
- *
- * @deprecated 4.4.0 will be removed in 6.0
- *             Use defined('_JEXEC') or die; to detect if the CMS is loaded correctly
- **/
-if (!defined('JPATH_PLATFORM')) {
-    define('JPATH_PLATFORM', __DIR__);
-}
-
 // Import the library loader if necessary
 if (!class_exists('JLoader')) {
     require_once JPATH_LIBRARIES . '/loader.php';
diff --git a/libraries/import.legacy.php b/libraries/import.legacy.php
index 60ea147aaa017..0a79a5d06722d 100644
--- a/libraries/import.legacy.php
+++ b/libraries/import.legacy.php
@@ -17,16 +17,6 @@
     E_USER_DEPRECATED
 );
 
-/**
- * Set the platform root path as a constant if necessary.
- *
- * @deprecated 4.4.0 will be removed in 6.0
- *             Use defined('_JEXEC') or die; to detect if the CMS is loaded correctly
- **/
-if (!defined('JPATH_PLATFORM')) {
-    define('JPATH_PLATFORM', __DIR__);
-}
-
 // Detect the native operating system type.
 $os = strtoupper(substr(PHP_OS, 0, 3));
 
diff --git a/libraries/import.php b/libraries/import.php
index cc87ee797a32f..020761674cb8d 100644
--- a/libraries/import.php
+++ b/libraries/import.php
@@ -17,16 +17,6 @@
     E_USER_DEPRECATED
 );
 
-/**
- * Set the platform root path as a constant if necessary.
- *
- * @deprecated 4.4.0 will be removed in 6.0
- *             Use defined('_JEXEC') or die; to detect if the CMS is loaded correctly
- **/
-if (!defined('JPATH_PLATFORM')) {
-    define('JPATH_PLATFORM', __DIR__);
-}
-
 // Detect the native operating system type.
 $os = strtoupper(substr(PHP_OS, 0, 3));
 
diff --git a/plugins/behaviour/compat/src/Extension/Compat.php b/plugins/behaviour/compat/src/Extension/Compat.php
index 7232a8866269a..ceecc2d2e6547 100644
--- a/plugins/behaviour/compat/src/Extension/Compat.php
+++ b/plugins/behaviour/compat/src/Extension/Compat.php
@@ -74,6 +74,12 @@ public function __construct(DispatcherInterface $dispatcher, array $config = [])
         if ($this->params->get('classes_aliases', '1')) {
             require_once \dirname(__DIR__) . '/classmap/classmap.php';
         }
+
+        /**
+         * Load the constant early as it is used in class files before the class itself is loaded.
+         * @deprecated 4.4.0 will be removed in 7.0
+         */
+        \defined('JPATH_PLATFORM') or \define('JPATH_PLATFORM', __DIR__);
     }
 
     /**
diff --git a/tests/Unit/bootstrap.php b/tests/Unit/bootstrap.php
index c506928078569..bc296b8d08349 100644
--- a/tests/Unit/bootstrap.php
+++ b/tests/Unit/bootstrap.php
@@ -35,13 +35,6 @@
     \define('JPATH_ROOT', JPATH_BASE);
 }
 
-/**
- * @deprecated 4.4.0 will be removed in 6.0
- **/
-if (!\defined('JPATH_PLATFORM')) {
-    \define('JPATH_PLATFORM', JPATH_BASE . DIRECTORY_SEPARATOR . 'libraries');
-}
-
 if (!\defined('JPATH_LIBRARIES')) {
     \define('JPATH_LIBRARIES', JPATH_BASE . DIRECTORY_SEPARATOR . 'libraries');
 }