Changing WooCommerce cart item names

The goal is to change the name of the item as it gets passed to our payment gateway, but leave it as-is for display on our product pages.

I've tried this in my functions.php:

function change_item_name( $item_name, $item ) {
    $item_name = 'mydesiredproductname';
    return $item_name;
}
add_filter( 'woocommerce_order_item_name', 'change_item_name', 10, 1 );

But it doesn't seem to be working for me. I feel like I should be passing in an actual item ID or something… I'm a little lost.

Any information on what I'm doing wrong here would be greatly appreciated.

The woocommerce_order_item_name filter hook is a front-end hook and is located in:

1) WooCommerce templates:

emails/plain/email-order-items.php
templates/order/order-details-item.php
templates/checkout/form-pay.php
templates/emails/email-order-items.php

2)WooCommerce Core file:

includes/class-wc-structured-data.php

Each of them has $item_name common first argument, and different for the other arguments.
See Here for more details…

You have 2 arguments set in your function (the second one is not correct for all templates) and you are declaring only one in your hook. I have tested the code below:

add_filter( 'woocommerce_order_item_name', 'change_orders_items_names', 10, 1 );
function change_orders_items_names( $item_name ) {
    $item_name = 'mydesiredproductname';
    return $item_name;
}

And it works on

Order-receive (Thank you) page,
email notifications
and My Account Orders > Single order details

But NOT in Cart, Checkout and Backend Order edit pages.

So if you need to make it work on Cart and Checkout, you should use other hooks like woocommerce_before_calculate_totals.
Then you can use WC_Product methods (setter and getters).

Here is your new code

add_action( 'woocommerce_before_calculate_totals', 'custom_cart_items_prices', 10, 1 );
function custom_cart_items_prices( $cart ) {

    if ( is_admin() && ! defined( 'DOING_AJAX' ) )
        return;

    if ( did_action( 'woocommerce_before_calculate_totals' ) >= 2 )
        return;

    // Loop through cart items
    foreach ( $cart->get_cart() as $cart_item ) {

        // Get an instance of the WC_Product object
        $product = $cart_item['data'];

        // Get the product name (Added Woocommerce 3+ compatibility)
        $original_name = method_exists( $product, 'get_name' ) ? $product->get_name() : $product->post->post_title;

        // SET THE NEW NAME
        $new_name = 'mydesiredproductname';

        // Set the new name (WooCommerce versions 2.5.x to 3+)
        if( method_exists( $product, 'set_name' ) )
            $product->set_name( $new_name );
        else
            $product->post->post_title = $new_name;
    }
}

Code goes in any php file of your active child theme (or theme) or also in any plugin php file.

Now you have change the names everywhere except on Shop archives and product pages…

This code is tested and works on WooCommerce 2.5+ and 3+

If you want to keep original item names in cart only you should add this conditional WooCommerce tag inside the function:
if( ! is_cart() ){
    // The code
}

This answer has been updated on August 1st 2017 to get a woocommerce compatibility prior versions…