diff --git a/res/composer-schema.json b/res/composer-schema.json
index 165ffcf85..04db6d3a3 100644
--- a/res/composer-schema.json
+++ b/res/composer-schema.json
@@ -565,6 +565,10 @@
                 "type": "string"
             }
         },
+        "default-branch": {
+            "type": ["boolean"],
+            "description": "Internal use only, do not specify this in composer.json. Indicates whether this version is the default branch of the linked VCS repository. Defaults to false."
+        },
         "abandoned": {
             "type": ["boolean", "string"],
             "description": "Indicates whether this package has been abandoned, it can be boolean or a package name/URL pointing to a recommended alternative. Defaults to false."
diff --git a/src/Composer/Package/Dumper/ArrayDumper.php b/src/Composer/Package/Dumper/ArrayDumper.php
index 84af84778..903997601 100644
--- a/src/Composer/Package/Dumper/ArrayDumper.php
+++ b/src/Composer/Package/Dumper/ArrayDumper.php
@@ -96,7 +96,7 @@ class ArrayDumper
         }
 
         if ($package->isDefaultBranch()) {
-            $data['default_branch'] = true;
+            $data['default-branch'] = true;
         }
 
         $data = $this->dumpValues($package, $keys, $data);
diff --git a/src/Composer/Package/Loader/ArrayLoader.php b/src/Composer/Package/Loader/ArrayLoader.php
index 314019e17..b2330e6e2 100644
--- a/src/Composer/Package/Loader/ArrayLoader.php
+++ b/src/Composer/Package/Loader/ArrayLoader.php
@@ -124,7 +124,7 @@ class ArrayLoader implements LoaderInterface
             $package->setInstallationSource($config['installation-source']);
         }
 
-        if (isset($config['default_branch']) && $config['default_branch'] === true) {
+        if (isset($config['default-branch']) && $config['default-branch'] === true) {
             $package->setIsDefaultBranch(true);
         }
 
@@ -368,7 +368,7 @@ class ArrayLoader implements LoaderInterface
             }
         }
 
-        if (isset($config['default_branch']) && $config['default_branch'] === true) {
+        if (isset($config['default-branch']) && $config['default-branch'] === true) {
             return VersionParser::DEV_MASTER_ALIAS;
         }
     }
diff --git a/src/Composer/Repository/VcsRepository.php b/src/Composer/Repository/VcsRepository.php
index d63623ccb..f3d7fe56b 100644
--- a/src/Composer/Repository/VcsRepository.php
+++ b/src/Composer/Repository/VcsRepository.php
@@ -231,8 +231,8 @@ class VcsRepository extends ArrayRepository implements ConfigurableRepositoryInt
                 $data['version'] = preg_replace('{[.-]?dev$}i', '', $data['version']);
                 $data['version_normalized'] = preg_replace('{(^dev-|[.-]?dev$)}i', '', $data['version_normalized']);
 
-                // make sure tag do not contain the default_branch marker
-                unset($data['default_branch']);
+                // make sure tag do not contain the default-branch marker
+                unset($data['default-branch']);
 
                 // broken package, version doesn't match tag
                 if ($data['version_normalized'] !== $parsedTag) {
@@ -337,9 +337,9 @@ class VcsRepository extends ArrayRepository implements ConfigurableRepositoryInt
                 $data['version'] = $version;
                 $data['version_normalized'] = $parsedBranch;
 
-                unset($data['default_branch']);
+                unset($data['default-branch']);
                 if ($driver->getRootIdentifier() === $branch) {
-                    $data['default_branch'] = true;
+                    $data['default-branch'] = true;
                 }
 
                 if ($isVeryVerbose) {
@@ -446,9 +446,9 @@ class VcsRepository extends ArrayRepository implements ConfigurableRepositoryInt
                 $this->io->overwriteError($msg, false);
             }
 
-            unset($cachedPackage['default_branch']);
+            unset($cachedPackage['default-branch']);
             if ($isDefaultBranch) {
-                $cachedPackage['default_branch'] = true;
+                $cachedPackage['default-branch'] = true;
             }
 
             if ($existingPackage = $this->findPackage($cachedPackage['name'], new Constraint('=', $cachedPackage['version_normalized']))) {