diff --git a/plugins/webp-uploads/helper.php b/plugins/webp-uploads/helper.php
index 312bd45d68..95fc15e18e 100644
--- a/plugins/webp-uploads/helper.php
+++ b/plugins/webp-uploads/helper.php
@@ -411,6 +411,17 @@ function webp_uploads_is_fallback_enabled(): bool {
 	return (bool) get_option( 'perflab_generate_webp_and_jpeg' );
 }
 
+/**
+ * Checks if the `perflab_generate_all_fallback_sizes` option is enabled.
+ *
+ * @since n.e.x.t
+ *
+ * @return bool Whether the option is enabled. Default is false.
+ */
+function webp_uploads_should_generate_all_fallback_sizes(): bool {
+	return (bool) get_option( 'perflab_generate_all_fallback_sizes', 0 );
+}
+
 /**
  * Retrieves the image URL for a specified MIME type from the attachment metadata.
  *
diff --git a/plugins/webp-uploads/hooks.php b/plugins/webp-uploads/hooks.php
index ce4c38a635..0e36bbca5a 100644
--- a/plugins/webp-uploads/hooks.php
+++ b/plugins/webp-uploads/hooks.php
@@ -806,3 +806,24 @@ function webp_uploads_opt_in_extra_image_sizes(): void {
 	}
 }
 add_action( 'plugins_loaded', 'webp_uploads_opt_in_extra_image_sizes' );
+
+/**
+ * Enables additional MIME type support for all image sizes based on the generate all fallback sizes settings.
+ *
+ * @since n.e.x.t
+ *
+ * @param array<string, bool> $allowed_sizes A map of image size names and whether they are allowed to have additional MIME types.
+ * @return array<string, bool> Modified map of image sizes with additional MIME type support.
+ */
+function webp_uploads_enable_additional_mime_type_support_for_all_sizes( array $allowed_sizes ): array {
+	if ( ! webp_uploads_should_generate_all_fallback_sizes() ) {
+		return $allowed_sizes;
+	}
+
+	foreach ( array_keys( $allowed_sizes ) as $size ) {
+		$allowed_sizes[ $size ] = true;
+	}
+
+	return $allowed_sizes;
+}
+add_filter( 'webp_uploads_image_sizes_with_additional_mime_type_support', 'webp_uploads_enable_additional_mime_type_support_for_all_sizes' );
diff --git a/plugins/webp-uploads/settings.php b/plugins/webp-uploads/settings.php
index c44d6c452a..cad0352e11 100644
--- a/plugins/webp-uploads/settings.php
+++ b/plugins/webp-uploads/settings.php
@@ -40,6 +40,18 @@ function webp_uploads_register_media_settings_field(): void {
 			'show_in_rest' => false,
 		)
 	);
+
+	// Add a setting to generate fallback images in all sizes including custom sizes.
+	register_setting(
+		'media',
+		'perflab_generate_all_fallback_sizes',
+		array(
+			'type'         => 'boolean',
+			'default'      => false,
+			'show_in_rest' => false,
+		)
+	);
+
 	// Add a setting to use the picture element.
 	register_setting(
 		'media',
@@ -96,6 +108,16 @@ function webp_uploads_add_media_settings_fields(): void {
 		array( 'class' => 'perflab-generate-webp-and-jpeg' )
 	);
 
+	// Add setting field for generating fallback images in all sizes including custom sizes.
+	add_settings_field(
+		'perflab_generate_all_fallback_sizes',
+		__( 'Generate all fallback image sizes', 'webp-uploads' ),
+		'webp_uploads_generate_all_fallback_sizes_callback',
+		'media',
+		'perflab_modern_image_format_settings',
+		array( 'class' => 'perflab-generate-fallback-all-sizes' )
+	);
+
 	// Add picture element support settings field.
 	add_settings_field(
 		'webp_uploads_use_picture_element',
@@ -178,6 +200,94 @@ function webp_uploads_generate_webp_jpeg_setting_callback(): void {
 	<?php
 }
 
+
+/**
+ * Renders the settings field for generating all fallback image sizes.
+ *
+ * @since n.e.x.t
+ */
+function webp_uploads_generate_all_fallback_sizes_callback(): void {
+	$all_fallback_sizes_enabled   = webp_uploads_should_generate_all_fallback_sizes();
+	$fallback_enabled             = webp_uploads_is_fallback_enabled();
+	$all_fallback_sizes_hidden_id = 'perflab_generate_all_fallback_sizes_hidden';
+
+	?>
+	<style>
+		#perflab_generate_all_fallback_sizes_fieldset.disabled label,
+		#perflab_generate_all_fallback_sizes_fieldset.disabled p {
+			opacity: 0.7;
+		}
+	</style>
+	<div id="perflab_generate_all_fallback_sizes_notice" class="notice notice-info inline" <?php echo $fallback_enabled ? 'hidden' : ''; ?>>
+		<p><?php esc_html_e( 'This setting requires fallback image output to be enabled.', 'webp-uploads' ); ?></p>
+	</div>
+	<div id="perflab_generate_all_fallback_sizes_fieldset" class="<?php echo ! $fallback_enabled ? 'disabled' : ''; ?>">
+		<label for="perflab_generate_all_fallback_sizes" id="perflab_generate_all_fallback_sizes_label">
+			<input
+				type="checkbox"
+				id="perflab_generate_all_fallback_sizes"
+				name="perflab_generate_all_fallback_sizes"
+				aria-describedby="perflab_generate_all_fallback_sizes_description"
+				value="1"
+				<?php checked( $all_fallback_sizes_enabled ); ?>
+				<?php disabled( ! $fallback_enabled ); ?>
+			>
+			<?php
+			/*
+			 * If the checkbox is disabled, but the option is enabled, include a hidden input to continue sending the
+			 * same value upon form submission.
+			 */
+			if ( ! $fallback_enabled && $all_fallback_sizes_enabled ) {
+				?>
+				<input
+					type="hidden"
+					id="<?php echo esc_attr( $all_fallback_sizes_hidden_id ); ?>"
+					name="perflab_generate_all_fallback_sizes"
+					value="1"
+				>
+				<?php
+			}
+			esc_html_e( 'Generate all fallback image sizes including custom sizes', 'webp-uploads' );
+			?>
+		</label>
+		<p class="description" id="perflab_generate_all_fallback_sizes_description"><?php esc_html_e( 'Enabling this option will generate all fallback image sizes including custom sizes. Note: uses even more storage space.', 'webp-uploads' ); ?></p>
+	</div>
+	<script>
+	( function ( allFallbackSizesHiddenId ) {
+		const fallbackCheckbox = document.getElementById( 'perflab_generate_webp_and_jpeg' );
+		const allFallbackSizesCheckbox = document.getElementById( 'perflab_generate_all_fallback_sizes' );
+		const allFallbackSizesFieldset = document.getElementById( 'perflab_generate_all_fallback_sizes_fieldset' );
+		const allFallbackSizesNotice = document.getElementById( 'perflab_generate_all_fallback_sizes_notice' );
+
+		function toggleAllFallbackSizes() {
+			const fallbackEnabled = fallbackCheckbox.checked;
+			allFallbackSizesFieldset.classList.toggle( 'disabled', ! fallbackEnabled );
+			allFallbackSizesCheckbox.disabled = ! fallbackEnabled;
+			allFallbackSizesNotice.hidden = fallbackEnabled;
+
+			// Remove or inject hidden input to preserve original setting value as needed.
+			if ( fallbackEnabled ) {
+				const hiddenInput = document.getElementById( allFallbackSizesHiddenId );
+				if ( hiddenInput ) {
+					hiddenInput.parentElement.removeChild( hiddenInput );
+				}
+			} else if ( allFallbackSizesCheckbox.checked && ! document.getElementById( allFallbackSizesHiddenId ) ) {
+				// The hidden input is only needed if the value was originally set (i.e., the checkbox enabled).
+				const hiddenInput = document.createElement( 'input' );
+				hiddenInput.type = 'hidden';
+				hiddenInput.id = allFallbackSizesHiddenId;
+				hiddenInput.name = allFallbackSizesCheckbox.name;
+				hiddenInput.value = allFallbackSizesCheckbox.value;
+				allFallbackSizesCheckbox.parentElement.insertBefore( hiddenInput, allFallbackSizesCheckbox.nextSibling );
+			}
+		}
+
+		fallbackCheckbox.addEventListener( 'change', toggleAllFallbackSizes );
+	} )( <?php echo wp_json_encode( $all_fallback_sizes_hidden_id ); ?> );
+	</script>
+	<?php
+}
+
 /**
  * Renders the settings field for the 'webp_uploads_use_picture_element' setting.
  *
diff --git a/plugins/webp-uploads/tests/test-load.php b/plugins/webp-uploads/tests/test-load.php
index 69ff75be07..8935827664 100644
--- a/plugins/webp-uploads/tests/test-load.php
+++ b/plugins/webp-uploads/tests/test-load.php
@@ -1045,4 +1045,46 @@ public function temp_filename(): string {
 
 		return $filename;
 	}
+
+	/**
+	 * Test that fallback images are generated for all sizes when the `perflab_generate_all_fallback_sizes` option is enabled.
+	 *
+	 * @dataProvider data_provider_supported_image_types
+	 *
+	 * @param string $image_type The image type.
+	 */
+	public function test_it_should_generate_fallback_images_for_all_sizes_when_generate_all_fallback_sizes_option_is_enabled( string $image_type ): void {
+		$mime_type = 'image/' . $image_type;
+
+		// Ensure the MIME type is supported; skip the test if not.
+		if ( ! webp_uploads_mime_type_supported( $mime_type ) ) {
+			$this->markTestSkipped( 'Mime type ' . $mime_type . ' is not supported.' );
+		}
+
+		// Register custom image sizes.
+		add_image_size( 'custom_size_1', 200, 200, true );
+		add_image_size( 'custom_size_2', 400, 400, true );
+
+		// Generate image output type and fallback image.
+		update_option( 'perflab_generate_webp_and_jpeg', true );
+
+		// Generate fallback images for all sizes.
+		update_option( 'perflab_generate_all_fallback_sizes', true );
+
+		$this->set_image_output_type( $image_type );
+
+		$attachment_id = self::factory()->attachment->create_upload_object( TESTS_PLUGIN_DIR . '/tests/data/images/leaves.jpg' );
+
+		// Clean up custom sizes.
+		remove_image_size( 'custom_size_1' );
+		remove_image_size( 'custom_size_2' );
+
+		// Verify that fallback images are generated for custom sizes.
+		foreach ( array( 'custom_size_1', 'custom_size_2' ) as $size_name ) {
+			$this->assertImageHasSizeSource( $attachment_id, $size_name, 'image/jpeg' );
+			$this->assertImageHasSizeSource( $attachment_id, $size_name, $mime_type );
+		}
+
+		wp_delete_attachment( $attachment_id );
+	}
 }
diff --git a/plugins/webp-uploads/uninstall.php b/plugins/webp-uploads/uninstall.php
index 46fba3c78d..8b3fae0aee 100644
--- a/plugins/webp-uploads/uninstall.php
+++ b/plugins/webp-uploads/uninstall.php
@@ -38,4 +38,5 @@
  */
 function webp_uploads_delete_plugin_option(): void {
 	delete_option( 'perflab_generate_webp_and_jpeg' );
+	delete_option( 'perflab_generate_all_fallback_sizes' );
 }