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). 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.

Keach was born in Savannah, Georgia, to Mary Cain (née Peckham), an actress, and Stacy Keach Sr., a theatre director, drama teacher, and actor with dozens of television and theatrical film credits billed as "Stacy Keach." The younger Keach was born with a cleft lip and a partial cleft of the hard palate, and he underwent numerous operations as a child. Throughout his adult life he has usually worn a mustache to hide the scars. He is now the honorary chairman of the Cleft Palate Foundation and advocates for insurance coverage for surgeries.

Keach graduated from Van Nuys High School in June 1959, where he was class president, attended the American Legion's Boys State summer program of California, then earned two BA degrees at the University of California, Berkeley (1963): one in English, the other in Dramatic Art. He earned a Master of Fine Arts at the Yale School of Drama in 1966 and was a Fulbright Scholar at the London Academy of Music and Dramatic Art.

While studying in London, Keach met Laurence Olivier, his acting hero.

Keach played the title role in MacBird!, an Off-Broadway anti-war satire by Barbara Garson staged at the Village Gate in 1966. In 1967, he was cast, again Off-Broadway, in George Tabori's The Niggerlovers with Morgan Freeman in his acting debut. To this day, Freeman credits Keach with teaching him the most about acting. In 1967, Keach also starred in We Bombed in New Haven, a play by Joseph Heller that premiered in New Haven at the Yale Repertory Theatre and later was produced on Broadway. Keach first appeared on Broadway in 1969 as Buffalo Bill in Indians by Arthur Kopit. Early in his career, he was credited as Stacy Keach Jr. to distinguish himself from his father. He played the lead actor in The Nude Paper Sermon, an avant-garde musical theatre piece for media presentation, commissioned by Nonesuch Records by composer Eric Salzman.