The Milan metro system has recently added support for Apple Pay. New contactless terminals in the underground subway system work with Visa and MasterCard credit and debit cards.
Although Apple branding isn’t seen on the terminals, the terminals have NFC and they will work with your iPhone and Apple Watch. You’ll need to add a card to Wallet beforehand, and since 18 banks in Italy support Apple Pay, chances are you’ll be supported.
A caveat: Contactless payment is only available on journeys that start on the underground subway, and you have to use the same device on entry and exit. The ATM page has further details.