William Gregory Magnussen (born April 20, 1985) is an American actor. He has been featured in the films Into the Woods (2014), Bridge of Spies (2015), Birth of the Dragon (2016), Game Night (2018), and Aladdin (2019), and has had supporting television roles in Get Shorty (2017) and Maniac (2018). In 2021, Magnussen starred in the sci-fi series Made for Love on HBO Max and appeared in the films The Many Saints of Newark and No Time to Die. In 2023, he starred as Rey "The King" Kingston in Spy Kids: Armageddon.

He has starred in Broadway and off-Broadway theatre productions, including 2013's Vanya and Sonia and Masha and Spike, for which he received a nomination for the Tony Award for Best Featured Actor in a Play.