diff --git a/includes/class-create-block-theme-api.php b/includes/class-create-block-theme-api.php
index a620fbb0..f7ff32fc 100644
--- a/includes/class-create-block-theme-api.php
+++ b/includes/class-create-block-theme-api.php
@@ -13,6 +13,7 @@
 require_once __DIR__ . '/create-theme/theme-readme.php';
 require_once __DIR__ . '/create-theme/theme-fonts.php';
 require_once __DIR__ . '/create-theme/theme-create.php';
+require_once __DIR__ . '/create-theme/theme-fluid-typography.php';
 
 /**
  * The api functionality of the plugin leveraged by the site editor UI.
diff --git a/includes/create-theme/theme-fluid-typography.php b/includes/create-theme/theme-fluid-typography.php
new file mode 100644
index 00000000..65fbe197
--- /dev/null
+++ b/includes/create-theme/theme-fluid-typography.php
@@ -0,0 +1,68 @@
+<?php
+
+class CBT_Theme_Fluid_Typography {
+
+	/**
+	 * Sync the size property with fluid.max in font sizes.
+	 * This ensures that when fluid typography is edited, the size property
+	 * stays in sync with the maximum fluid size.
+	 *
+	 * @param array $data The theme.json data to process.
+	 * @return array The processed data with synced sizes.
+	 */
+	public static function sync_fluid_typography_sizes( $data ) {
+		if ( ! isset( $data['settings']['typography']['fontSizes'] ) ) {
+			return $data;
+		}
+
+		$font_sizes = $data['settings']['typography']['fontSizes'];
+
+		foreach ( $font_sizes as $key => $font_size ) {
+			// Check if this font size has fluid settings
+			if ( isset( $font_size['fluid'] ) && is_array( $font_size['fluid'] ) ) {
+				// If fluid.max is set, sync it with size
+				if ( isset( $font_size['fluid']['max'] ) ) {
+					$font_sizes[ $key ]['size'] = $font_size['fluid']['max'];
+				}
+			}
+		}
+
+		$data['settings']['typography']['fontSizes'] = $font_sizes;
+
+		return $data;
+	}
+
+	/**
+	 * Hook into the global styles REST API to sync fluid typography sizes.
+	 */
+	public static function init() {
+		add_filter( 'rest_pre_insert_wp_global_styles', array( __CLASS__, 'sync_global_styles_fluid_typography' ), 10, 2 );
+	}
+
+	/**
+	 * Sync fluid typography in global styles before saving.
+	 *
+	 * @param stdClass $prepared_post The prepared post object.
+	 * @return stdClass The modified prepared post object.
+	 */
+	public static function sync_global_styles_fluid_typography( $prepared_post ) {
+		if ( ! isset( $prepared_post->post_content ) ) {
+			return $prepared_post;
+		}
+
+		$data = json_decode( $prepared_post->post_content, true );
+
+		if ( ! is_array( $data ) ) {
+			return $prepared_post;
+		}
+
+		$data = self::sync_fluid_typography_sizes( $data );
+
+		$prepared_post->post_content = wp_json_encode( $data );
+
+		return $prepared_post;
+	}
+}
+
+// Initialize the fluid typography sync on plugins_loaded
+add_action( 'plugins_loaded', array( 'CBT_Theme_Fluid_Typography', 'init' ) );
diff --git a/tests/test-fluid-typography.php b/tests/test-fluid-typography.php
new file mode 100644
index 00000000..b8ebb0ce
--- /dev/null
+++ b/tests/test-fluid-typography.php
@@ -0,0 +1,129 @@
+<?php
+/**
+ * @package Create_Block_Theme
+ * @group fluid-typography
+ */
+class Test_Create_Block_Theme_Fluid_Typography extends WP_UnitTestCase {
+
+	/**
+	 * Test that fluid typography sizes are synced correctly
+	 */
+	public function test_sync_fluid_typography_sizes() {
+		$data = array(
+			'settings' => array(
+				'typography' => array(
+					'fontSizes' => array(
+						array(
+							'slug'  => 'custom-fluid',
+							'name'  => 'Custom Fluid',
+							'fluid' => array(
+								'min' => '1.5rem',
+								'max' => '5.63rem',
+							),
+							'size'  => '3rem', // Old value, should be updated
+						),
+						array(
+							'slug' => 'normal',
+							'name' => 'Normal',
+							'size' => '1rem', // No fluid, should remain unchanged
+						),
+					),
+				),
+			),
+		);
+
+		$result = CBT_Theme_Fluid_Typography::sync_fluid_typography_sizes( $data );
+
+		// Verify the fluid size was synced
+		$this->assertEquals(
+			'5.63rem',
+			$result['settings']['typography']['fontSizes'][0]['size'],
+			'Fluid typography size should be synced with fluid.max'
+		);
+
+		// Verify the non-fluid size remained unchanged
+		$this->assertEquals(
+			'1rem',
+			$result['settings']['typography']['fontSizes'][1]['size'],
+			'Non-fluid typography size should remain unchanged'
+		);
+	}
+
+	/**
+	 * Test that global styles filter works correctly
+	 */
+	public function test_sync_global_styles_fluid_typography() {
+		$prepared_post               = new stdClass();
+		$prepared_post->post_content = wp_json_encode(
+			array(
+				'settings' => array(
+					'typography' => array(
+						'fontSizes' => array(
+							array(
+								'slug'  => 'custom-fluid',
+								'name'  => 'Custom Fluid',
+								'fluid' => array(
+									'min' => '1.5rem',
+									'max' => '5.63rem',
+								),
+								'size'  => '3rem',
+							),
+						),
+					),
+				),
+			)
+		);
+
+		$result = CBT_Theme_Fluid_Typography::sync_global_styles_fluid_typography( $prepared_post );
+
+		$data = json_decode( $result->post_content, true );
+
+		$this->assertEquals(
+			'5.63rem',
+			$data['settings']['typography']['fontSizes'][0]['size'],
+			'Global styles should have synced fluid typography sizes'
+		);
+	}
+
+	/**
+	 * Test that missing typography doesn't cause errors
+	 */
+	public function test_sync_with_no_typography() {
+		$data = array(
+			'settings' => array(),
+		);
+
+		$result = CBT_Theme_Fluid_Typography::sync_fluid_typography_sizes( $data );
+
+		$this->assertArrayNotHasKey( 'typography', $result['settings'] );
+		$this->assertEqualsCanonicalizing( array(), $result['settings'] );
+	}
+
+	/**
+	 * Test that missing fontSizes doesn't cause errors
+	 */
+	public function test_sync_with_no_font_sizes() {
+		$data = array(
+			'settings' => array(
+				'typography' => array(),
+			),
+		);
+
+		$result = CBT_Theme_Fluid_Typography::sync_fluid_typography_sizes( $data );
+
+		$this->assertArrayNotHasKey( 'fontSizes', $result['settings']['typography'] );
+	}
+
+	/**
+	 * Test that invalid JSON in global styles filter is handled gracefully
+	 */
+	public function test_sync_global_styles_with_invalid_json() {
+		$prepared_post               = new stdClass();
+		$prepared_post->post_content = 'invalid json';
+
+		$result = CBT_Theme_Fluid_Typography::sync_global_styles_fluid_typography( $prepared_post );
+
+		// Should return the prepared post unchanged
+		$this->assertEquals( 'invalid json', $result->post_content );
+	}
+}