Guy Alexander Masterson (Mastroianni; born August 10, 1961) is a British actor, writer, theatre director, producer, and playwright. He is known for his solo performances of Under Milk Wood, Animal Farm, and Shylock by Gareth Armstrong.

At the Edinburgh Fringe Festival, Masterson was involved in productions such as Twelve Angry Men (2003), The Odd Couple (2005), Morecambe (2009), and The Shark Is Broken (2019). Morecambe and The Shark is Broken both transferred to London's West End, with the latter opening on Broadway on August 10, 2023.