diff --git a/src/Core/Manifest/ManifestFileFinder.php b/src/Core/Manifest/ManifestFileFinder.php
index 75763e4ed65..f534469807c 100644
--- a/src/Core/Manifest/ManifestFileFinder.php
+++ b/src/Core/Manifest/ManifestFileFinder.php
@@ -25,11 +25,6 @@ class ManifestFileFinder extends FileFinder
     const TESTS_DIR = 'tests';
     const VENDOR_DIR = 'vendor';
 
-    /**
-     * @deprecated 4.4.0:5.0.0 Use global `RESOURCES_DIR` instead.
-     */
-    const RESOURCES_DIR = RESOURCES_DIR;
-
     protected static $default_options = [
         'include_themes' => false,
         'ignore_tests' => true,
diff --git a/src/Core/Manifest/Module.php b/src/Core/Manifest/Module.php
index 1872c763747..bedc9a9b616 100644
--- a/src/Core/Manifest/Module.php
+++ b/src/Core/Manifest/Module.php
@@ -147,20 +147,6 @@ public function getShortName()
         return basename($this->path ?? '');
     }
 
-    /**
-     * Name of the resource directory where vendor resources should be exposed as defined by the `extra.resources-dir`
-     * key in the composer file. A blank string will be returned if the key is undefined.
-     *
-     * Only applicable when reading the composer file for the main project.
-     * @return string
-     */
-    public function getResourcesDir()
-    {
-        return isset($this->composerData['extra']['resources-dir'])
-            ? $this->composerData['extra']['resources-dir']
-            : '';
-    }
-
     /**
      * Get base path for this module
      *
diff --git a/src/includes/constants.php b/src/includes/constants.php
index 24968564d93..7116fd828e3 100644
--- a/src/includes/constants.php
+++ b/src/includes/constants.php
@@ -28,7 +28,7 @@
  * - PUBLIC_PATH: Absolute path to webroot, e.g. "/var/www/project/public"
  * - THIRDPARTY_DIR: Path relative to webroot, e.g. "framework/thirdparty"
  * - THIRDPARTY_PATH: Absolute filepath, e.g. "/var/www/my-webroot/framework/thirdparty"
- * - RESOURCES_DIR: Name of the directory where vendor assets will be exposed, e.g. "_resources"
+ * - RESOURCES_DIR: Name of the directory where vendor assets will be exposed, i.e. "_resources"
  */
 
 require_once __DIR__ . '/functions.php';
@@ -218,15 +218,5 @@
 
 // Define the resource dir constant that will be use to exposed vendor assets
 if (!defined('RESOURCES_DIR')) {
-    $project = new SilverStripe\Core\Manifest\Module(BASE_PATH, BASE_PATH);
-    $resourcesDir = $project->getResourcesDir() ?: 'resources';
-    if (preg_match('/^[_\-a-z0-9]+$/i', $resourcesDir ?? '')) {
-        define('RESOURCES_DIR', $resourcesDir);
-    } else {
-        throw new LogicException(sprintf(
-            'Resources dir error: "%s" is not a valid resources directory name. Update the ' .
-            '`extra.resources-dir` key in your composer.json file',
-            $resourcesDir
-        ));
-    }
+    define('RESOURCES_DIR', '_resources');
 }
diff --git a/tests/php/Core/Manifest/ModuleTest.php b/tests/php/Core/Manifest/ModuleTest.php
index f4ea2db5a97..36420c5202f 100644
--- a/tests/php/Core/Manifest/ModuleTest.php
+++ b/tests/php/Core/Manifest/ModuleTest.php
@@ -2,26 +2,11 @@
 
 namespace SilverStripe\Core\Tests\Manifest;
 
-use SilverStripe\Control\Director;
 use SilverStripe\Core\Manifest\Module;
 use SilverStripe\Dev\SapphireTest;
 
 class ModuleTest extends SapphireTest
 {
-    public function testUnsetResourcesDir()
-    {
-        $path = __DIR__ . '/fixtures/ss-projects/withoutCustomResourcesDir';
-        $module = new Module($path, $path);
-        $this->assertEquals('', $module->getResourcesDir());
-    }
-
-    public function testResourcesDir()
-    {
-        $path = __DIR__ . '/fixtures/ss-projects/withCustomResourcesDir';
-        $module = new Module($path, $path);
-        $this->assertEquals('customised-resources-dir', $module->getResourcesDir());
-    }
-
     /**
      * @dataProvider ciConfigProvider
      * @param string $fixture The folder containing our test composer file
diff --git a/tests/php/Core/Manifest/fixtures/ss-projects/withCustomResourcesDir/composer.json b/tests/php/Core/Manifest/fixtures/ss-projects/withCustomResourcesDir/composer.json
deleted file mode 100644
index ee9e3ac5f3a..00000000000
--- a/tests/php/Core/Manifest/fixtures/ss-projects/withCustomResourcesDir/composer.json
+++ /dev/null
@@ -1,29 +0,0 @@
-{
-    "name": "silverstripe/ss44",
-    "type": "silverstripe-project",
-    "description": "Fake project using SS 4.4",
-    "homepage": "https://www.silverstripe.org",
-    "license": "BSD-3-Clause",
-    "require": {
-        "silverstripe/recipe-cms": "4.4.x-dev as 4.4.0"
-    },
-    "extra": {
-        "project-files-installed": [
-            "app/.htaccess",
-            "app/_config.php",
-            "app/_config/mysite.yml",
-            "app/src/Page.php",
-            "app/src/PageController.php"
-        ],
-        "public-files-installed": [
-            ".htaccess",
-            "index.php",
-            "install-frameworkmissing.html",
-            "install.php",
-            "web.config"
-        ],
-        "resources-dir": "customised-resources-dir"
-    },
-    "prefer-stable": true,
-    "minimum-stability": "dev"
-}
diff --git a/tests/php/Core/Manifest/fixtures/ss-projects/withoutCustomResourcesDir/composer.json b/tests/php/Core/Manifest/fixtures/ss-projects/withoutCustomResourcesDir/composer.json
deleted file mode 100644
index 7f9bd60a8d6..00000000000
--- a/tests/php/Core/Manifest/fixtures/ss-projects/withoutCustomResourcesDir/composer.json
+++ /dev/null
@@ -1,28 +0,0 @@
-{
-    "name": "silverstripe/ss44",
-    "type": "silverstripe-project",
-    "description": "Fake project using SS 4.4",
-    "homepage": "https://www.silverstripe.org",
-    "license": "BSD-3-Clause",
-    "require": {
-        "silverstripe/recipe-cms": "4.4.x-dev as 4.4.0"
-    },
-    "extra": {
-        "project-files-installed": [
-            "app/.htaccess",
-            "app/_config.php",
-            "app/_config/mysite.yml",
-            "app/src/Page.php",
-            "app/src/PageController.php"
-        ],
-        "public-files-installed": [
-            ".htaccess",
-            "index.php",
-            "install-frameworkmissing.html",
-            "install.php",
-            "web.config"
-        ]
-    },
-    "prefer-stable": true,
-    "minimum-stability": "dev"
-}