diff --git a/tests/phpunit/tests/functions/isBlogInstalled.php b/tests/phpunit/tests/functions/isBlogInstalled.php
new file mode 100644
index 0000000000000..5d4373095a29f
--- /dev/null
+++ b/tests/phpunit/tests/functions/isBlogInstalled.php
@@ -0,0 +1,107 @@
+<?php
+
+/**
+ * Tests for the is_blog_installed function.
+ *
+ * @group functions
+ *
+ * @covers ::is_blog_installed
+ */
+
+class Test_Functions_isBlogInstalled extends WP_UnitTestCase {
+
+	private $original_wpdb;
+
+
+	public function set_up(): void {
+		parent::set_up();
+		global $wpdb;
+
+		$this->original_wpdb = $wpdb;
+		// Create a mock for the wpdb object
+		$wpdb = $this->createMock( 'wpdb' );
+
+		// Set the global $wpdb to the mock
+		$this->setUpMockedWpdb( $wpdb );
+	}
+
+	public function tear_down(): void {
+		global $wpdb;
+		$wpdb = $this->original_wpdb;
+
+		wp_cache_set( 'is_blog_installed', false );
+
+		parent::tear_down();
+	}
+
+	public function test_should_return_true_by_default() {
+		$this->assertTrue( is_blog_installed() );
+	}
+
+	// Test case 1: Returns true when siteurl exists in the database
+	public function test_should_return_true_when_siteurl_exists() {
+		global $wpdb;
+
+		// Mock the get_var to return 'http://example.com' for siteurl
+		$wpdb->method( 'get_var' )->willReturn( 'http://example.com' );
+
+		$result = is_blog_installed();
+
+		$this->assertTrue( $result );
+	}
+
+	// Test case 2: Returns false when siteurl is empty
+	public function test_should_return_false_when_siteurl_is_empty() {
+		global $wpdb;
+
+		$filter_callback = function ( $alloptions ) {
+			$alloptions['siteurl'] = '';
+			return $alloptions;
+		};
+
+		add_filter(
+			'pre_wp_load_alloptions',
+			$filter_callback,
+			10,
+			1
+		);
+
+		$result = is_blog_installed();
+
+		remove_filter( 'pre_wp_load_alloptions', $filter_callback );
+
+		$this->assertFalse( $result );
+	}
+
+	// Test case 3: Returns true when WP_REPAIRING constant is defined
+	public function test_should_return_true_when_wp_repairing_is_defined() {
+
+		if ( ! defined( 'WP_REPAIRING' ) ) {
+			define( 'WP_REPAIRING', true );
+		}
+
+		$result = is_blog_installed();
+
+		$this->assertTrue( $result );
+	}
+
+	// Test case 4: Cache behavior - Returns cached result
+	public function test_should_use_cache_once_installed_status_is_checked() {
+		global $wpdb;
+
+		wp_cache_set( 'is_blog_installed', true );
+
+		$result = is_blog_installed();
+
+		$this->assertTrue( $result );
+
+		$result = is_blog_installed();
+		$this->assertTrue( $result );
+	}
+
+	// Helper to mock the global wpdb instance
+	private function setUpMockedWpdb( $wpdb_mock ) {
+		global $wpdb;
+		$wpdb = $wpdb_mock;
+	}
+}