diff --git a/features/package-install.feature b/features/package-install.feature
index 495d77af..fd91d45c 100644
--- a/features/package-install.feature
+++ b/features/package-install.feature
@@ -298,6 +298,102 @@ Feature: Install WP-CLI packages
       Success: Uninstalled package.
       """
 
+  @github-api
+  Scenario: Install a package from a Git URL without .git suffix
+    Given an empty directory
+
+    When I run `wp package install https://github.com/wp-cli/google-sitemap-generator-cli`
+    Then STDOUT should contain:
+      """
+      Installing package wp-cli/google-sitemap-generator-cli
+      """
+    And STDOUT should contain:
+      """
+      Registering https://github.com/wp-cli/google-sitemap-generator-cli.git as a VCS repository...
+      """
+    And STDOUT should contain:
+      """
+      Success: Package installed.
+      """
+
+    When I run `wp package uninstall wp-cli/google-sitemap-generator-cli`
+    Then STDOUT should contain:
+      """
+      Success: Uninstalled package.
+      """
+
+  @github-api
+  Scenario: Install a package from a Git URL without .git suffix but with a version suffix
+    Given an empty directory
+
+    When I run `wp package install https://github.com/wp-cli/google-sitemap-generator-cli:dev-main`
+    Then STDOUT should contain:
+      """
+      Installing package wp-cli/google-sitemap-generator-cli (dev-main)
+      """
+    And STDOUT should contain:
+      """
+      Registering https://github.com/wp-cli/google-sitemap-generator-cli.git as a VCS repository...
+      """
+    And STDOUT should contain:
+      """
+      Success: Package installed.
+      """
+
+    When I run `wp package uninstall wp-cli/google-sitemap-generator-cli`
+    Then STDOUT should contain:
+      """
+      Success: Uninstalled package.
+      """
+
+  @github-api
+  Scenario: Install a package from a GitHub SSH URL without .git suffix
+    Given an empty directory
+
+    When I run `wp package install git@github.com:wp-cli/google-sitemap-generator-cli`
+    Then STDOUT should contain:
+      """
+      Installing package wp-cli/google-sitemap-generator-cli
+      """
+    And STDOUT should contain:
+      """
+      Registering git@github.com:wp-cli/google-sitemap-generator-cli.git as a VCS repository...
+      """
+    And STDOUT should contain:
+      """
+      Success: Package installed.
+      """
+
+    When I run `wp package uninstall wp-cli/google-sitemap-generator-cli`
+    Then STDOUT should contain:
+      """
+      Success: Uninstalled package.
+      """
+
+  @gitlab-api
+  Scenario: Install a package from a GitLab URL without .git suffix and nested groups
+    Given an empty directory
+
+    When I run `wp package install https://gitlab.com/wp-cli/wp-cli-test/test-command`
+    Then STDOUT should contain:
+      """
+      Installing package wp-cli-test/test-command
+      """
+    And STDOUT should contain:
+      """
+      Registering https://gitlab.com/wp-cli/wp-cli-test/test-command.git as a VCS repository...
+      """
+    And STDOUT should contain:
+      """
+      Success: Package installed.
+      """
+
+    When I run `wp package uninstall wp-cli-test/test-command`
+    Then STDOUT should contain:
+      """
+      Success: Uninstalled package.
+      """
+
   @github-api
   Scenario: Install a package from a Git URL with mixed-case git name but lowercase composer.json name
     Given an empty directory
diff --git a/src/Package_Command.php b/src/Package_Command.php
index 12e784a9..8e8c04b9 100644
--- a/src/Package_Command.php
+++ b/src/Package_Command.php
@@ -222,6 +222,8 @@ public function install( $args, $assoc_args ) {
 		$git_package = false;
 		$dir_package = false;
 		$version     = '';
+		// Append .git suffix for GitHub/GitLab URLs that are missing it.
+		$package_name = $this->maybe_add_git_suffix( $package_name );
 		// Parse version suffix from a git URL (e.g. https://github.com/vendor/package.git:dev-main).
 		if ( preg_match( '#^(.+\.git):([^:]+)$#', $package_name, $url_version_matches ) ) {
 			$package_name = $url_version_matches[1];
@@ -229,7 +231,11 @@ public function install( $args, $assoc_args ) {
 		}
 		if ( $this->is_git_repository( $package_name ) ) {
 			if ( '' === $version ) {
-				$version = "dev-{$this->get_github_default_branch( $package_name, $insecure )}";
+				if ( preg_match( '#^(?:https?://github\.com/|git@github\.com:)#i', $package_name ) ) {
+					$version = "dev-{$this->get_github_default_branch( $package_name, $insecure )}";
+				} else {
+					$version = 'dev-master';
+				}
 			}
 			$git_package = $package_name;
 			$matches     = [];
@@ -1296,6 +1302,25 @@ private function is_git_repository( $package ) {
 		return '.git' === strtolower( substr( $package, -4, 4 ) );
 	}
 
+	/**
+	 * Appends the .git suffix to GitHub or GitLab URLs that are missing it.
+	 *
+	 * @param string $package_name Package name or URL to normalize.
+	 * @return string Normalized package name or URL.
+	 */
+	private function maybe_add_git_suffix( $package_name ) {
+		// Already has .git suffix (possibly followed by :version).
+		if ( preg_match( '#\.git(?::[^:]+)?$#i', $package_name ) ) {
+			return $package_name;
+		}
+		// Append .git for GitHub/GitLab HTTPS or SSH URLs, preserving any :version suffix.
+		// Pattern: (https?://github.com/<user>/<repo>[...subgroups...] or git@github.com:<user>/<repo>[...subgroups...])(:version)?
+		if ( preg_match( '#^((?:https?://(?:github|gitlab)\.com/|git@(?:github|gitlab)\.com:)[^/:]+(?:/[^/:]+)*)(:.*)?$#i', $package_name, $matches ) ) {
+			return $matches[1] . '.git' . ( $matches[2] ?? '' );
+		}
+		return $package_name;
+	}
+
 	/**
 	 * Checks that `$package_name` matches the name in composer.json at Github.com, and return corrected value if not.
 	 *