diff --git a/src/wp-includes/canonical.php b/src/wp-includes/canonical.php
index 6b8c17c07d55a..51135d66491db 100644
--- a/src/wp-includes/canonical.php
+++ b/src/wp-includes/canonical.php
@@ -9,6 +9,31 @@
  * @since 2.3.0
  */
 
+/**
+ * Redirects HTTP requests to HTTPS when SSL is forced for the site.
+ *
+ * @since x.x.x
+ * @access private
+ */
+function _wp_force_ssl_redirect() {
+	if ( ! force_ssl() || is_ssl() || ! isset( $_SERVER['HTTP_HOST'], $_SERVER['REQUEST_URI'] ) ) {
+		return;
+	}
+
+	if ( isset( $_SERVER['REQUEST_METHOD'] ) && ! in_array( strtoupper( $_SERVER['REQUEST_METHOD'] ), array( 'GET', 'HEAD' ), true ) ) {
+		return;
+	}
+
+	if ( str_starts_with( $_SERVER['REQUEST_URI'], 'http' ) ) {
+		$redirect = set_url_scheme( wp_unslash( $_SERVER['REQUEST_URI'] ), 'https' );
+	} else {
+		$redirect = 'https://' . wp_unslash( $_SERVER['HTTP_HOST'] ) . wp_unslash( $_SERVER['REQUEST_URI'] );
+	}
+
+	wp_safe_redirect( $redirect );
+	exit;
+}
+
 /**
  * Redirects incoming links to the proper URL based on the site url.
  *
diff --git a/src/wp-includes/class-wp-recovery-mode-cookie-service.php b/src/wp-includes/class-wp-recovery-mode-cookie-service.php
index a2ee34a723aa8..d62bd50453759 100644
--- a/src/wp-includes/class-wp-recovery-mode-cookie-service.php
+++ b/src/wp-includes/class-wp-recovery-mode-cookie-service.php
@@ -47,10 +47,12 @@ public function set_cookie() {
 
 		$expire = time() + $length;
 
-		setcookie( RECOVERY_MODE_COOKIE, $value, $expire, COOKIEPATH, COOKIE_DOMAIN, is_ssl(), true );
+		$secure = is_ssl() || force_ssl();
+
+		setcookie( RECOVERY_MODE_COOKIE, $value, $expire, COOKIEPATH, COOKIE_DOMAIN, $secure, true );
 
 		if ( COOKIEPATH !== SITECOOKIEPATH ) {
-			setcookie( RECOVERY_MODE_COOKIE, $value, $expire, SITECOOKIEPATH, COOKIE_DOMAIN, is_ssl(), true );
+			setcookie( RECOVERY_MODE_COOKIE, $value, $expire, SITECOOKIEPATH, COOKIE_DOMAIN, $secure, true );
 		}
 	}
 
diff --git a/src/wp-includes/default-constants.php b/src/wp-includes/default-constants.php
index acfc878fb7138..0da2f5935e321 100644
--- a/src/wp-includes/default-constants.php
+++ b/src/wp-includes/default-constants.php
@@ -347,17 +347,25 @@ function wp_cookie_constants() {
  * @since 3.0.0
  */
 function wp_ssl_constants() {
+	/**
+	 * @since x.x.x
+	 */
+	if ( ! defined( 'FORCE_SSL' ) ) {
+		define( 'FORCE_SSL', false );
+	}
+	force_ssl( FORCE_SSL );
+
 	/**
 	 * @since 2.6.0
 	 */
 	if ( ! defined( 'FORCE_SSL_ADMIN' ) ) {
-		if ( 'https' === parse_url( get_option( 'siteurl' ), PHP_URL_SCHEME ) ) {
+		if ( force_ssl() || 'https' === parse_url( get_option( 'siteurl' ), PHP_URL_SCHEME ) ) {
 			define( 'FORCE_SSL_ADMIN', true );
 		} else {
 			define( 'FORCE_SSL_ADMIN', false );
 		}
 	}
-	force_ssl_admin( FORCE_SSL_ADMIN );
+	force_ssl_admin( force_ssl() || FORCE_SSL_ADMIN );
 
 	/**
 	 * @since 2.6.0
diff --git a/src/wp-includes/default-filters.php b/src/wp-includes/default-filters.php
index 4e0b74a8d22c2..d9597852a6faf 100644
--- a/src/wp-includes/default-filters.php
+++ b/src/wp-includes/default-filters.php
@@ -659,7 +659,9 @@
 add_action( 'wp_footer', 'wp_enqueue_stored_styles', 1 );
 
 add_action( 'wp_default_styles', 'wp_default_styles' );
+add_filter( 'script_loader_src', '_wp_force_ssl_url' );
 add_filter( 'style_loader_src', 'wp_style_loader_src', 10, 2 );
+add_filter( 'style_loader_src', '_wp_force_ssl_url', 11 );
 
 add_action( 'wp_head', 'wp_enqueue_img_auto_sizes_contain_css_fix', 0 ); // Must run before wp_print_auto_sizes_contain_css_fix().
 add_action( 'wp_head', 'wp_print_auto_sizes_contain_css_fix', 1 ); // Retained for backwards-compatibility. Unhooked by wp_enqueue_img_auto_sizes_contain_css_fix().
@@ -687,6 +689,7 @@
 add_action( 'change_locale', 'create_initial_taxonomies' );
 
 // Canonical.
+add_action( 'template_redirect', '_wp_force_ssl_redirect', 0 );
 add_action( 'template_redirect', 'redirect_canonical' );
 add_action( 'template_redirect', 'wp_redirect_admin_locations', 1000 );
 
diff --git a/src/wp-includes/functions.php b/src/wp-includes/functions.php
index 64dce4a159755..e844539506cce 100644
--- a/src/wp-includes/functions.php
+++ b/src/wp-includes/functions.php
@@ -6360,6 +6360,26 @@ function force_ssl_admin( $force = null ) {
 	return $forced;
 }
 
+/**
+ * Determines whether to force SSL for the entire site.
+ *
+ * @since x.x.x
+ *
+ * @param string|bool|null $force Optional. Whether to force SSL. Default null.
+ * @return bool True if forced, false if not forced.
+ */
+function force_ssl( $force = null ) {
+	static $forced = false;
+
+	if ( ! is_null( $force ) ) {
+		$old_forced = $forced;
+		$forced     = (bool) $force;
+		return $old_forced;
+	}
+
+	return $forced;
+}
+
 /**
  * Guesses the URL for the site.
  *
diff --git a/src/wp-includes/https-migration.php b/src/wp-includes/https-migration.php
index 1232154bbced1..4be75e2da8c93 100644
--- a/src/wp-includes/https-migration.php
+++ b/src/wp-includes/https-migration.php
@@ -18,8 +18,8 @@
  * @return bool True if insecure URLs should replaced, false otherwise.
  */
 function wp_should_replace_insecure_home_url() {
-	$should_replace_insecure_home_url = wp_is_using_https()
-		&& get_option( 'https_migration_required' )
+	$should_replace_insecure_home_url = ( force_ssl() || wp_is_using_https() )
+		&& ( force_ssl() || get_option( 'https_migration_required' ) )
 		// For automatic replacement, both 'home' and 'siteurl' need to not only use HTTPS, they also need to be using
 		// the same domain.
 		&& wp_parse_url( home_url(), PHP_URL_HOST ) === wp_parse_url( site_url(), PHP_URL_HOST );
diff --git a/src/wp-includes/link-template.php b/src/wp-includes/link-template.php
index 223d6b5548fc6..76b67c446112e 100644
--- a/src/wp-includes/link-template.php
+++ b/src/wp-includes/link-template.php
@@ -3469,7 +3469,7 @@ function get_home_url( $blog_id = null, $path = '', $scheme = null ) {
 	}
 
 	if ( ! in_array( $scheme, array( 'http', 'https', 'relative' ), true ) ) {
-		if ( is_ssl() ) {
+		if ( is_ssl() || force_ssl() ) {
 			$scheme = 'https';
 		} else {
 			$scheme = parse_url( $url, PHP_URL_SCHEME );
@@ -3790,7 +3790,7 @@ function network_home_url( $path = '', $scheme = null ) {
 	$orig_scheme     = $scheme;
 
 	if ( ! in_array( $scheme, array( 'http', 'https', 'relative' ), true ) ) {
-		$scheme = is_ssl() ? 'https' : 'http';
+		$scheme = is_ssl() || force_ssl() ? 'https' : 'http';
 	}
 
 	if ( 'relative' === $scheme ) {
@@ -3931,11 +3931,11 @@ function set_url_scheme( $url, $scheme = null ) {
 	$orig_scheme = $scheme;
 
 	if ( ! $scheme ) {
-		$scheme = is_ssl() ? 'https' : 'http';
+		$scheme = is_ssl() || force_ssl() ? 'https' : 'http';
 	} elseif ( 'admin' === $scheme || 'login' === $scheme || 'login_post' === $scheme || 'rpc' === $scheme ) {
-		$scheme = is_ssl() || force_ssl_admin() ? 'https' : 'http';
+		$scheme = is_ssl() || force_ssl() || force_ssl_admin() ? 'https' : 'http';
 	} elseif ( 'http' !== $scheme && 'https' !== $scheme && 'relative' !== $scheme ) {
-		$scheme = is_ssl() ? 'https' : 'http';
+		$scheme = is_ssl() || force_ssl() ? 'https' : 'http';
 	}
 
 	$url = trim( $url );
@@ -3965,6 +3965,37 @@ function set_url_scheme( $url, $scheme = null ) {
 	return apply_filters( 'set_url_scheme', $url, $scheme, $orig_scheme );
 }
 
+/**
+ * Converts a local URL to HTTPS when SSL is forced.
+ *
+ * @since x.x.x
+ * @access private
+ *
+ * @param string $url URL to check.
+ * @return string HTTPS URL when local SSL is forced, otherwise the original URL.
+ */
+function _wp_force_ssl_url( $url ) {
+	if ( ! force_ssl() || ! is_string( $url ) || ! preg_match( '#^(https?:)?//#i', $url ) ) {
+		return $url;
+	}
+
+	$host = wp_parse_url( $url, PHP_URL_HOST );
+	if ( ! $host ) {
+		return $url;
+	}
+
+	$host       = strtolower( $host );
+	$site_hosts = array_filter(
+		array(
+			wp_parse_url( home_url( '', 'http' ), PHP_URL_HOST ),
+			wp_parse_url( site_url( '', 'http' ), PHP_URL_HOST ),
+		)
+	);
+	$site_hosts = array_map( 'strtolower', $site_hosts );
+
+	return in_array( $host, $site_hosts, true ) ? set_url_scheme( $url, 'https' ) : $url;
+}
+
 /**
  * Retrieves the URL to the user's dashboard.
  *
diff --git a/src/wp-includes/pluggable.php b/src/wp-includes/pluggable.php
index f659186b9a70c..2a2e2678db737 100644
--- a/src/wp-includes/pluggable.php
+++ b/src/wp-includes/pluggable.php
@@ -1093,11 +1093,11 @@ function wp_set_auth_cookie( $user_id, $remember = false, $secure = '', $token =
 		}
 
 		if ( '' === $secure ) {
-			$secure = is_ssl();
+			$secure = is_ssl() || force_ssl();
 		}
 
 		// Front-end cookie is secure when the auth cookie is secure and the site's home URL uses HTTPS.
-		$secure_logged_in_cookie = $secure && 'https' === parse_url( get_option( 'home' ), PHP_URL_SCHEME );
+		$secure_logged_in_cookie = $secure && ( force_ssl() || 'https' === parse_url( get_option( 'home' ), PHP_URL_SCHEME ) );
 
 		/**
 		 * Filters whether the auth cookie should only be sent over HTTPS.
@@ -1278,7 +1278,7 @@ function is_user_logged_in() {
 	 * @since 1.5.0
 	 */
 	function auth_redirect() {
-		$secure = ( is_ssl() || force_ssl_admin() );
+		$secure = ( is_ssl() || force_ssl() || force_ssl_admin() );
 
 		/**
 		 * Filters whether to use a secure authentication redirect.
diff --git a/src/wp-login.php b/src/wp-login.php
index abedea82c3589..bfe7af0deacd1 100644
--- a/src/wp-login.php
+++ b/src/wp-login.php
@@ -792,6 +792,8 @@ function wp_login_viewport_meta() {
 			$secure = false;
 		}
 
+		$secure = $secure || force_ssl();
+
 		setcookie( 'wp-postpass_' . COOKIEHASH, $hasher->HashPassword( wp_unslash( $_POST['post_password'] ) ), $expire, COOKIEPATH, COOKIE_DOMAIN, $secure );
 
 		wp_safe_redirect( $redirect_to );
@@ -942,7 +944,7 @@ function wp_login_viewport_meta() {
 
 		if ( isset( $_GET['key'] ) && isset( $_GET['login'] ) ) {
 			$value = sprintf( '%s:%s', wp_unslash( $_GET['login'] ), wp_unslash( $_GET['key'] ) );
-			setcookie( $rp_cookie, $value, 0, $rp_path, COOKIE_DOMAIN, is_ssl(), true );
+			setcookie( $rp_cookie, $value, 0, $rp_path, COOKIE_DOMAIN, is_ssl() || force_ssl(), true );
 
 			wp_safe_redirect( remove_query_arg( array( 'key', 'login' ) ) );
 			exit;
@@ -961,7 +963,7 @@ function wp_login_viewport_meta() {
 		}
 
 		if ( ! $user || is_wp_error( $user ) ) {
-			setcookie( $rp_cookie, ' ', time() - YEAR_IN_SECONDS, $rp_path, COOKIE_DOMAIN, is_ssl(), true );
+			setcookie( $rp_cookie, ' ', time() - YEAR_IN_SECONDS, $rp_path, COOKIE_DOMAIN, is_ssl() || force_ssl(), true );
 
 			if ( $user && $user->get_error_code() === 'expired_key' ) {
 				wp_redirect( site_url( 'wp-login.php?action=lostpassword&error=expiredkey' ) );
@@ -1491,7 +1493,7 @@ function wp_login_viewport_meta() {
 		if ( isset( $_COOKIE[ $rp_cookie ] ) && is_string( $_COOKIE[ $rp_cookie ] ) ) {
 			$user_login      = sanitize_user( strtok( wp_unslash( $_COOKIE[ $rp_cookie ] ), ':' ) );
 			list( $rp_path ) = explode( '?', wp_unslash( $_SERVER['REQUEST_URI'] ) );
-			setcookie( $rp_cookie, ' ', time() - YEAR_IN_SECONDS, $rp_path, COOKIE_DOMAIN, is_ssl(), true );
+			setcookie( $rp_cookie, ' ', time() - YEAR_IN_SECONDS, $rp_path, COOKIE_DOMAIN, is_ssl() || force_ssl(), true );
 		}
 
 		login_header( __( 'Log In' ), '', $errors );
diff --git a/tests/phpunit/tests/functions/forceSslAdmin.php b/tests/phpunit/tests/functions/forceSslAdmin.php
index e0432c382e959..3792192673bd2 100644
--- a/tests/phpunit/tests/functions/forceSslAdmin.php
+++ b/tests/phpunit/tests/functions/forceSslAdmin.php
@@ -1,19 +1,21 @@
 <?php
 /**
- * Test cases for the `force_ssl_admin()` function.
+ * Test cases for the `force_ssl_admin()` and `force_ssl()` functions.
  *
  * @since 6.8.0
  *
  * @group functions
  *
  * @covers ::force_ssl_admin
+ * @covers ::force_ssl
  */
 class Tests_Functions_ForceSslAdmin extends WP_UnitTestCase {
 
 	public function set_up() {
 		parent::set_up();
-		// Reset the `$forced` static variable before each test.
+		// Reset the `$forced` static variables before each test.
 		force_ssl_admin( false );
+		force_ssl( false );
 	}
 
 	/**
@@ -48,4 +50,23 @@ public function data_force_ssl_admin() {
 			'integer 0'        => array( 0, false ),
 		);
 	}
+
+	/**
+	 * Tests that force_ssl() returns expected values based on various inputs.
+	 *
+	 * @ticket 28521
+	 *
+	 * @dataProvider data_force_ssl_admin
+	 *
+	 * @param mixed $input    The input value to test.
+	 * @param bool  $expected The expected result for subsequent calls.
+	 */
+	public function test_force_ssl( $input, $expected ) {
+		// The first call always returns the previous value.
+		$this->assertFalse( force_ssl( $input ), 'First call did not return the expected value' );
+
+		// Call again to check subsequent behavior.
+		$this->assertSame( $expected, force_ssl( $input ), 'Subsequent call did not return the expected value' );
+		$this->assertSame( $expected, force_ssl(), 'Getter did not return the expected value' );
+	}
 }
diff --git a/tests/phpunit/tests/https-migration.php b/tests/phpunit/tests/https-migration.php
index ae66738603e13..1d0938a34579d 100644
--- a/tests/phpunit/tests/https-migration.php
+++ b/tests/phpunit/tests/https-migration.php
@@ -5,6 +5,11 @@
  */
 class Tests_HTTPS_Migration extends WP_UnitTestCase {
 
+	public function tear_down() {
+		force_ssl( false );
+		parent::tear_down();
+	}
+
 	/**
 	 * @ticket 51437
 	 */
@@ -25,6 +30,11 @@ public function test_wp_should_replace_insecure_home_url() {
 		update_option( 'https_migration_required', '1' );
 		$this->assertTrue( wp_should_replace_insecure_home_url() );
 
+		// Should return true when SSL is forced, even without the HTTPS migration flag.
+		update_option( 'https_migration_required', false );
+		force_ssl( true );
+		$this->assertTrue( wp_should_replace_insecure_home_url() );
+
 		// Should be overridable via filter.
 		add_filter( 'wp_should_replace_insecure_home_url', '__return_false' );
 		$this->assertFalse( wp_should_replace_insecure_home_url() );
diff --git a/tests/phpunit/tests/url.php b/tests/phpunit/tests/url.php
index 3734891eb19ac..2840606fb4f10 100644
--- a/tests/phpunit/tests/url.php
+++ b/tests/phpunit/tests/url.php
@@ -356,6 +356,48 @@ public function test_set_url_scheme() {
 		force_ssl_admin( $forced_admin );
 	}
 
+	/**
+	 * @ticket 28521
+	 *
+	 * @covers ::set_url_scheme
+	 * @covers ::get_home_url
+	 * @covers ::_wp_force_ssl_url
+	 */
+	public function test_force_ssl_sets_local_url_schemes_to_https() {
+		$forced_ssl       = force_ssl();
+		$forced_admin     = force_ssl_admin();
+		$server_https     = $_SERVER['HTTPS'] ?? null;
+		$_SERVER['HTTPS'] = 'off';
+		force_ssl_admin( false );
+		force_ssl( true );
+
+		try {
+			$http_home  = home_url( '/foo/', 'http' );
+			$https_home = home_url( '/foo/', 'https' );
+
+			$this->assertSame( $https_home, set_url_scheme( $http_home ) );
+			$this->assertSame( $https_home, set_url_scheme( $http_home, 'admin' ) );
+			$this->assertSame( $https_home, home_url( '/foo/' ) );
+			$this->assertSame( $https_home, network_home_url( '/foo/' ) );
+			$this->assertSame( $http_home, home_url( '/foo/', 'http' ) );
+			$this->assertSame( $https_home, _wp_force_ssl_url( $http_home ) );
+			$this->assertSame( 'https://example.org/foo/', _wp_force_ssl_url( '//example.org/foo/' ) );
+			$this->assertSame( 'http://example.com/foo/', _wp_force_ssl_url( 'http://example.com/foo/' ) );
+			$this->assertSame( '/foo/', _wp_force_ssl_url( '/foo/' ) );
+
+			force_ssl( false );
+			$this->assertSame( $http_home, _wp_force_ssl_url( $http_home ) );
+		} finally {
+			force_ssl( $forced_ssl );
+			force_ssl_admin( $forced_admin );
+			if ( null === $server_https ) {
+				unset( $_SERVER['HTTPS'] );
+			} else {
+				$_SERVER['HTTPS'] = $server_https;
+			}
+		}
+	}
+
 	/**
 	 * @covers ::get_adjacent_post
 	 */