diff --git a/wp-modules/pattern-data-handlers/pattern-data-handlers.php b/wp-modules/pattern-data-handlers/pattern-data-handlers.php
index d32f876e2..72cc1facd 100644
--- a/wp-modules/pattern-data-handlers/pattern-data-handlers.php
+++ b/wp-modules/pattern-data-handlers/pattern-data-handlers.php
@@ -314,6 +314,9 @@ function update_pattern( $pattern ) {
 		FS_CHMOD_FILE
 	);
 
+	// Put a flag in the database to indicate this theme has been modified by PM.
+	update_option( 'pm_mod_' . get_stylesheet(), true );
+
 	// TO DO: Fix issue with needing to "Save twice" on the frontend, because the pattern files are cached on the first save, making images on disk incorrect.
 
 	return $pattern_file_created;
@@ -331,6 +334,9 @@ function delete_pattern( string $pattern_name ): bool {
 	$result        = $wp_filesystem && $wp_filesystem->exists( $pattern_path ) && $wp_filesystem->delete( $pattern_path );
 	tree_shake_theme_images( $wp_filesystem, 'copy_dir' );
 
+	// Put a flag in the database to indicate this theme has been modified by PM.
+	update_option( 'pm_mod_' . get_stylesheet(), true );
+
 	return $result;
 }
 
diff --git a/wp-modules/prevent-theme-updates/prevent-theme-updates.php b/wp-modules/prevent-theme-updates/prevent-theme-updates.php
new file mode 100644
index 000000000..7ccf03085
--- /dev/null
+++ b/wp-modules/prevent-theme-updates/prevent-theme-updates.php
@@ -0,0 +1,44 @@
+<?php
+/**
+ * Module Name: Prevent Theme Updates
+ * Description: This module will prevent a theme from being updated if Pattern Manager has modified it.
+ * Namespace: PreventThemeUpdates
+ *
+ * @package pattern-manager
+ */
+
+declare(strict_types=1);
+
+namespace PatternManager\PreventThemeUpdates;
+
+// Exit if accessed directly.
+if ( ! defined( 'ABSPATH' ) ) {
+	exit;
+}
+
+/**
+ * Prevent a theme update if it's been modified by Pattern Manager.
+ *
+ * @param mixed  $update_themes_transient_data  Value of site transient.
+ * @param string $transient                     Transient name.
+ * @return string
+ */
+function block_theme_updates_if_modified_by_pm( $update_themes_transient_data ) {
+
+	// No update object exists. Return early.
+	if ( empty( $update_themes_transient_data ) || ! isset( $update_themes_transient_data->response ) ) {
+		return $update_themes_transient_data;
+	}
+
+	// Loop through each theme that has an update available.
+	foreach ( $update_themes_transient_data->response as $theme_slug => $theme_with_update_available ) {
+		$theme_has_been_modified_by_pm = get_option( 'pm_mod_' . $theme_slug );
+
+		if ( $theme_has_been_modified_by_pm ) {
+			unset( $update_themes_transient_data->response[ $theme_slug ] );
+		}
+	}
+
+	return $update_themes_transient_data;
+}
+add_filter( 'site_transient_update_themes', __NAMESPACE__ . '\block_theme_updates_if_modified_by_pm', 10 );
diff --git a/wp-modules/prevent-theme-updates/tests/PreventThemeUpdateTest.php b/wp-modules/prevent-theme-updates/tests/PreventThemeUpdateTest.php
new file mode 100644
index 000000000..0d3bc6a8f
--- /dev/null
+++ b/wp-modules/prevent-theme-updates/tests/PreventThemeUpdateTest.php
@@ -0,0 +1,95 @@
+<?php
+/**
+ * Class PatternManagerPreventThemeUpdateTest
+ *
+ * @package pattern-manager
+ */
+
+/**
+ * Test this module's functions.
+ */
+class PatternManagerPreventThemeUpdateTest extends WP_UnitTestCase {
+
+	/**
+	 * Test that when themes do not have PM mods, their updates are not prevented.
+	 */
+	public function testDoNotBlockThemesWithNoPmMods() {
+		$value               = new stdClass();
+		$value->last_checked = 1678397389;
+		$value->checked      = array(
+			'twentytwentythree' => 1.1,
+			'twentytwentytwo'   => 1.4,
+		);
+		$value->response     = array(
+			'twentytwentythree' => array(
+				'theme'        => 'twentytwentythree',
+				'new_version'  => 1.0,
+				'url'          => 'https://wordpress.org/themes/twentytwentythree/',
+				'package'      => 'https://downloads.wordpress.org/theme/twentytwentythree.1.0.zip',
+				'requires'     => 6.1,
+				'requires_php' => 5.6,
+			),
+		);
+		$value->no_update    = array(
+			'twentytwentytwo' => array(
+				'theme'        => 'twentytwentytwo',
+				'new_version'  => 1.3,
+				'url'          => 'https://wordpress.org/themes/twentytwentytwo/',
+				'package'      => 'https://downloads.wordpress.org/theme/twentytwentytwo.1.3.zip',
+				'requires'     => 5.9,
+				'requires_php' => 5.6,
+			),
+		);
+		$value->translations = array();
+
+		$expected = $value;
+
+		$result = \PatternManager\PreventThemeUpdates\block_theme_updates_if_modified_by_pm( $value, 'update_themes' );
+
+		$this->assertEquals( $expected, $result );
+	}
+
+
+	/**
+	 * Test that when themes do have PM mods, their updates are prevented.
+	 */
+	public function testBlockThemesWithPmMods() {
+		update_option( 'pm_mod_twentytwentythree', true );
+
+		$value               = new stdClass();
+		$value->last_checked = 1678397389;
+		$value->checked      = array(
+			'twentytwentythree' => 1.1,
+			'twentytwentytwo'   => 1.4,
+		);
+		$value->response     = array(
+			'twentytwentythree' => array(
+				'theme'        => 'twentytwentythree',
+				'new_version'  => 1.0,
+				'url'          => 'https://wordpress.org/themes/twentytwentythree/',
+				'package'      => 'https://downloads.wordpress.org/theme/twentytwentythree.1.0.zip',
+				'requires'     => 6.1,
+				'requires_php' => 5.6,
+			),
+		);
+		$value->no_update    = array(
+			'twentytwentytwo' => array(
+				'theme'        => 'twentytwentytwo',
+				'new_version'  => 1.3,
+				'url'          => 'https://wordpress.org/themes/twentytwentytwo/',
+				'package'      => 'https://downloads.wordpress.org/theme/twentytwentytwo.1.3.zip',
+				'requires'     => 5.9,
+				'requires_php' => 5.6,
+			),
+		);
+		$value->translations = array();
+
+		$expected           = $value;
+		$expected->response = array();
+
+		$result = \PatternManager\PreventThemeUpdates\block_theme_updates_if_modified_by_pm( $value, 'update_themes' );
+
+		$this->assertEquals( $expected, $result );
+	}
+
+}