Armand Douglas Hammer (born August 28, 1986) is an American actor. He began his acting career with guest appearances in several television series. His first leading role was as Billy Graham in the 2008 film Billy: The Early Years and Hammer gained wider recognition for portraying the twins Cameron and Tyler Winklevoss in David Fincher's biopic The Social Network (2010).

Hammer portrayed Clyde Tolson in the biopic J. Edgar (2011), played the titular character in the western The Lone Ranger (2013), and starred as Illya Kuryakin in the action film The Man from U.N.C.L.E. (2015). In 2017, he starred in Luca Guadagnino's romantic drama Call Me by Your Name, for which he received a nomination for the Golden Globe for Best Supporting Actor. The following year, Hammer portrayed Martin D. Ginsburg in the biopic On the Basis of Sex (2018). On Broadway, he starred in a production of Straight White Men in 2018.

In 2021, multiple women came forward with claims of abuse against Hammer. Hammer denied the allegations, which derailed his acting career; he abandoned several future projects and was dropped by his talent agency and publicist. After an investigation into Hammer by the LA County District Attorney and the LAPD, they declined to pursue criminal charges against him, citing insufficient evidence. Hammer returned to acting in 2024 after a nearly 3-year hiatus from the industry.