Walter Stacy Keach Jr. (born June 2, 1941) is an American actor, active in theatre, film and television since the 1960s. Keach first distinguished himself in Off-Broadway productions and remains a prominent figure in American theatre across his career, particularly as a noted Shakespearean. He is the recipient of several theatrical accolades: four Drama Desk Awards, two Helen Hayes Awards and two Obie Awards for Distinguished Performance by an Actor. He was nominated for a Tony Award for Best Actor in a Play for his performance in Arthur Kopit's 1969 production of Indians.

In film, he garnered critical acclaim for his portrayal of a washed-up boxer in the John Huston film Fat City (1972) and appeared as Sergeant Stedenko in Cheech & Chong's films Up in Smoke (1978) and Nice Dreams (1981).[4] His other notable film credits include Brewster McCloud (1970), Doc (1971), The Life and Times of Judge Roy Bean (1972), The New Centurions (1972), Luther (1974), Slave of the Cannibal God (1978), The Ninth Configuration (1980), The Long Riders (1980), Roadgames (1981), Batman: Mask of the Phantasm (1993), Escape from L.A. (1996), American History X (1998), The Bourne Legacy (2012) and Nebraska (2013).

Keach is known to television audiences for his portrayal of private detective Mike Hammer in television movies and on the television series Mickey Spillane's Mike Hammer (1984–1987), for which he was nominated for a Golden Globe, as Ken Titus on the sitcom Titus (2000–2002) and as the narrator of the crime documentary series American Greed (2007–present). He also had a main cast role on the sitcom Man with a Plan (2017–2020) and recurring roles on series such as Prison Break (2005–2007), Two and a Half Men (2010), Blue Bloods (2016–2024) and The Blacklist (2019–2023). He won a Golden Globe and was nominated for an Primetime Emmy Award for playing Ernest Hemingway on the television miniseries Hemingway (1988).

He is an inductee of the Theatre Hall of Fame and was honored with a star on the Hollywood Walk of Fame in 2019. He is the son of theatre director Stacy Keach Sr., and the older brother of actor James Keach.