diff --git a/CHANGELOG-v3.md b/CHANGELOG-v3.md
index bcafc4128c5..3c1604c091c 100644
--- a/CHANGELOG-v3.md
+++ b/CHANGELOG-v3.md
@@ -3,6 +3,7 @@
 ## Unreleased
 
 ### Added
+- Added support for a `CRAFT_LICENSE_KEY` PHP constant, which can be set to the project’s license key, taking precedence over the `license.key` file.
 - Added `craft\helpers\Stringy::getLangSpecificCharsArray()`.
 - Added `craft\web\View::setRegisteredAssetBundles()`.
 - Added `craft\web\View::setRegisteredJsFiles()`.
diff --git a/bootstrap/bootstrap.php b/bootstrap/bootstrap.php
index 7dca9818b30..a3ac72ac6ef 100644
--- a/bootstrap/bootstrap.php
+++ b/bootstrap/bootstrap.php
@@ -110,37 +110,40 @@
 // Validate the paths
 // -----------------------------------------------------------------------------
 
-// Validate permissions on the license key file path (default config/) and storage/
-if (defined('CRAFT_LICENSE_KEY_PATH')) {
-    $licensePath = dirname(CRAFT_LICENSE_KEY_PATH);
-    $licenseKeyName = basename(CRAFT_LICENSE_KEY_PATH);
-} else {
-    $licensePath = $configPath;
-    $licenseKeyName = 'license.key';
-}
+if (!defined('CRAFT_LICENSE_KEY')) {
+    // Validate permissions on the license key file path (default config/) and storage/
+    if (defined('CRAFT_LICENSE_KEY_PATH')) {
+        $licensePath = dirname(CRAFT_LICENSE_KEY_PATH);
+        $licenseKeyName = basename(CRAFT_LICENSE_KEY_PATH);
+    } else {
+        $licensePath = $configPath;
+        $licenseKeyName = 'license.key';
+    }
 
-// Make sure the license folder exists.
-if (!is_dir($licensePath) && !file_exists($licensePath)) {
-    $createFolder($licensePath);
-}
+    // Make sure the license folder exists.
+    if (!is_dir($licensePath) && !file_exists($licensePath)) {
+        $createFolder($licensePath);
+    }
 
-$ensureFolderIsReadable($licensePath);
+    $ensureFolderIsReadable($licensePath);
 
-if ($appType === 'web') {
-    $licenseFullPath = $licensePath.'/'.$licenseKeyName;
+    if ($appType === 'web') {
+        $licenseFullPath = $licensePath.'/'.$licenseKeyName;
 
-    // If the license key doesn't exist yet, make sure the folder is readable and we can write a temp one.
-    if (!file_exists($licenseFullPath)) {
-        // Try and write out a temp license key file.
-        @file_put_contents($licenseFullPath, 'temp');
+        // If the license key doesn't exist yet, make sure the folder is readable and we can write a temp one.
+        if (!file_exists($licenseFullPath)) {
+            // Try and write out a temp license key file.
+            @file_put_contents($licenseFullPath, 'temp');
 
-        // See if it worked.
-        if (!file_exists($licenseFullPath) || (file_exists($licenseFullPath) && file_get_contents($licenseFullPath) !== 'temp')) {
-            exit($licensePath.' isn\'t writable by PHP. Please fix that.');
+            // See if it worked.
+            if (!file_exists($licenseFullPath) || (file_exists($licenseFullPath) && file_get_contents($licenseFullPath) !== 'temp')) {
+                exit($licensePath.' isn\'t writable by PHP. Please fix that.');
+            }
         }
     }
 }
 
+
 $ensureFolderIsReadable($storagePath, true);
 
 // Create the storage/runtime/ folder if it doesn't already exist
diff --git a/src/controllers/DashboardController.php b/src/controllers/DashboardController.php
index 6276d32f40d..df5a63163dd 100644
--- a/src/controllers/DashboardController.php
+++ b/src/controllers/DashboardController.php
@@ -361,9 +361,8 @@ public function actionSendSupportRequest(): Response
             }
 
             // License key
-            $licenseKeyPath = Craft::$app->getPath()->getLicenseKeyPath();
-            if (is_file($licenseKeyPath)) {
-                $zip->addFile($licenseKeyPath, 'license.key');
+            if (($licenseKey = App::licenseKey()) !== null) {
+                $zip->addFromString('license.key', $licenseKey);
             }
 
             // Logs
diff --git a/src/helpers/App.php b/src/helpers/App.php
index 89ab2368dd9..549e20e98e1 100644
--- a/src/helpers/App.php
+++ b/src/helpers/App.php
@@ -168,19 +168,20 @@ public static function maxPowerCaptain()
      */
     public static function licenseKey()
     {
-        $path = Craft::$app->getPath()->getLicenseKeyPath();
+        if (defined('CRAFT_LICENSE_KEY')) {
+            $licenseKey = CRAFT_LICENSE_KEY;
+        } else {
+            $path = Craft::$app->getPath()->getLicenseKeyPath();
 
-        // Check to see if the key exists and it's not a temp one.
-        if (!is_file($path)) {
-            return null;
-        }
+            // Check to see if the key exists
+            if (!is_file($path)) {
+                return null;
+            }
 
-        $contents = file_get_contents($path);
-        if (empty($contents) || $contents === 'temp') {
-            return null;
+            $licenseKey = file_get_contents($path);
         }
 
-        $licenseKey = trim(preg_replace('/[\r\n]+/', '', $contents));
+        $licenseKey = trim(preg_replace('/[\r\n]+/', '', $licenseKey));
 
         if (strlen($licenseKey) !== 250) {
             return null;
diff --git a/src/helpers/Cp.php b/src/helpers/Cp.php
index 484c4b6f982..2435ace1a6c 100644
--- a/src/helpers/Cp.php
+++ b/src/helpers/Cp.php
@@ -81,20 +81,28 @@ public static function alerts(string $path = null, bool $fetch = false): array
             // Domain mismatch?
             if ($licenseKeyStatus === LicenseKeyStatus::Mismatched) {
                 $licensedDomain = Craft::$app->getCache()->get('licensedDomain');
+                $domainLink = '<a href="http://'.$licensedDomain.'" target="_blank">'.$licensedDomain.'</a>';
+
+                if (defined('CRAFT_LICENSE_KEY')) {
+                    $message = Craft::t('app', 'The license key in use belongs to {domain}', [
+                        'domain' => $domainLink
+                    ]);
+                } else {
+                    $keyPath = Craft::$app->getPath()->getLicenseKeyPath();
+
+                    // If the license key path starts with the root project path, trim the project path off
+                    $rootPath = Craft::getAlias('@root');
+                    if (strpos($keyPath, $rootPath.'/') === 0) {
+                        $keyPath = substr($keyPath, strlen($rootPath) + 1);
+                    }
 
-                $keyPath = Craft::$app->getPath()->getLicenseKeyPath();
-
-                // If the license key path starts with the root project path, trim the project path off
-                $rootPath = Craft::getAlias('@root');
-                if (strpos($keyPath, $rootPath.'/') === 0) {
-                    $keyPath = substr($keyPath, strlen($rootPath) + 1);
+                    $message = Craft::t('app', 'The license located at {file} belongs to {domain}.', [
+                        'file' => $keyPath,
+                        'domain' => $domainLink
+                    ]);
                 }
 
-                $alerts[] = Craft::t('app', 'The license located at {file} belongs to {domain}.', [
-                        'file' => $keyPath,
-                        'domain' => '<a href="http://'.$licensedDomain.'" target="_blank">'.$licensedDomain.'</a>'
-                    ]).
-                    ' <a class="go" href="https://craftcms.com/support/resolving-mismatched-licenses">'.Craft::t('app', 'Learn more').'</a>';
+                $alerts[] = $message.' <a class="go" href="https://craftcms.com/support/resolving-mismatched-licenses">'.Craft::t('app', 'Learn more').'</a>';
             }
 
             // Any plugin issues?
diff --git a/src/services/Api.php b/src/services/Api.php
index a4d941c7ba4..5d17ae6f204 100644
--- a/src/services/Api.php
+++ b/src/services/Api.php
@@ -450,7 +450,7 @@ protected function headers(): array
         }
 
         // Craft license
-        $headers['X-Craft-License'] = App::licenseKey() ?? '🙏';
+        $headers['X-Craft-License'] = App::licenseKey() ?? (defined('CRAFT_LICENSE_KEY') ? '😱' : '🙏');
 
         // plugin info
         $pluginLicenses = [];