diff --git a/composer.lock b/composer.lock
index 58295fc987d77ad79d238f680c03166dedc99f9b..0b5b2bceab7302fab128aaa10d80e3c116c0d48f 100644
--- a/composer.lock
+++ b/composer.lock
@@ -453,7 +453,7 @@
         },
         {
             "name": "drupal/core",
-            "version": "9.0.x-dev",
+            "version": "9.1.x-dev",
             "dist": {
                 "type": "path",
                 "url": "core",
@@ -699,7 +699,7 @@
         },
         {
             "name": "drupal/core-project-message",
-            "version": "9.0.x-dev",
+            "version": "9.1.x-dev",
             "dist": {
                 "type": "path",
                 "url": "composer/Plugin/ProjectMessage",
@@ -729,7 +729,7 @@
         },
         {
             "name": "drupal/core-vendor-hardening",
-            "version": "9.0.x-dev",
+            "version": "9.1.x-dev",
             "dist": {
                 "type": "path",
                 "url": "composer/Plugin/VendorHardening",
diff --git a/composer/Metapackage/CoreRecommended/composer.json b/composer/Metapackage/CoreRecommended/composer.json
index 31e54857efdf18a22728929ef9a34672ecef622c..b3530f08f5de0ba58b7a2c5ed6f355e3e165b860 100644
--- a/composer/Metapackage/CoreRecommended/composer.json
+++ b/composer/Metapackage/CoreRecommended/composer.json
@@ -7,7 +7,7 @@
         "webflo/drupal-core-strict": "*"
     },
     "require": {
-        "drupal/core": "9.0.x-dev",
+        "drupal/core": "9.1.x-dev",
         "asm89/stack-cors": "1.3.0",
         "composer/installers": "v1.8.0",
         "composer/semver": "1.5.1",
diff --git a/composer/Metapackage/PinnedDevDependencies/composer.json b/composer/Metapackage/PinnedDevDependencies/composer.json
index 987f92a6ea37d785f14c62ae31353c1dec11b9a4..555cd7e69e442689201c6d158b855da7012515a6 100644
--- a/composer/Metapackage/PinnedDevDependencies/composer.json
+++ b/composer/Metapackage/PinnedDevDependencies/composer.json
@@ -7,7 +7,7 @@
         "webflo/drupal-core-require-dev": "*"
     },
     "require": {
-        "drupal/core": "9.0.x-dev",
+        "drupal/core": "9.1.x-dev",
         "behat/mink": "v1.8.1",
         "behat/mink-browserkit-driver": "v1.3.4",
         "behat/mink-goutte-driver": "v1.2.1",
diff --git a/core/lib/Drupal.php b/core/lib/Drupal.php
index eb155efb0e9da4f59198ae677ca551a3af982ceb..461fda85470692cbd5f6e90e686d20c9369a2033 100644
--- a/core/lib/Drupal.php
+++ b/core/lib/Drupal.php
@@ -80,7 +80,7 @@ class Drupal {
   /**
    * The current system version.
    */
-  const VERSION = '9.0.0-dev';
+  const VERSION = '9.1.0-dev';
 
   /**
    * Core API compatibility.