Viggo Peter Mortensen Jr. (; Danish: [viːko ˈmɒːtn̩sn̩]; born October 20, 1958) is a Danish-American actor, author, musician, photographer, poet, and painter. Born in New York to a Danish father and American mother, he was a resident of Venezuela and Argentina during his childhood. He is the recipient of various accolades including a Screen Actors Guild Award and has been nominated for three Academy Awards, three BAFTA Awards, and four Golden Globe Awards.

Mortensen made his film debut in a small role in Peter Weir's 1985 thriller Witness starring Harrison Ford and has appeared in several notable films since, including The Indian Runner (1991), Carlito's Way (1993), Crimson Tide (1995), Daylight (1996), The Portrait of a Lady (1996), G.I. Jane (1997), Psycho (1998), A Perfect Murder (1998), A Walk on the Moon (1999), and 28 Days (2000).

Mortensen received international attention in the early 2000s with his role as Aragorn in the epic fantasy adventure trilogy The Lord of the Rings (2001–2003). In 2005, Mortensen won critical acclaim for David Cronenberg's crime thriller A History of Violence. Two years later, another Cronenberg film, Eastern Promises (2007), earned him further critical acclaim and a nomination for the Academy Award for Best Actor. A third teaming with Cronenberg in A Dangerous Method (2011) resulted in a nomination for the Golden Globe Award for Best Supporting Actor – Motion Picture. Other well-received films include Appaloosa (2008) and Far from Men (2014). Further Academy Award nominations came for his leading roles in Captain Fantastic (2016) and Green Book (2018).

Aside from acting, Mortensen's other artistic pursuits include fine arts, photography, poetry, and music. In 2002, he founded the Perceval Press to publish the works of little-known artists and authors.