Marcus Dell Gastineau (b. November 20, 1956 in Ardmore, Oklahoma) is a former American football player.

He played defensive end for Arizona State University then East Central University in Ada, Oklahoma. He became that school's first-ever NFL draft pick, selected in the second round by the New York Jets. Gastineau made the Pro Bowl five straight seasons (1981-85) and finished his 10-year career with 107½ sacks. His best season was 1984, when his record-setting 22 sacks earned him NFL defensive player of the year honors.

Gastineau seemed poised to return to his old self, leading the AFC in sacks seven weeks into the 1988 season. However, he shocked the football world by announcing his retirement to spend more time with actress Brigitte Nielsen, to whom he had previously announced his engagement, even though he was still married to his first wife, Lisa; he and Nielsen later had a son.

In 1991, Gastineau pursued a career in boxing. His first fight was against Derrick Dukes. He won with a first round knockout. Dukes, a professional wrestler, later admitted he took a dive. TV newsmagazine show 60 Minutes interviewed several others that fought Gastineau and were told to take dives so that it made Gastineau look good. His career ended in 1996 when he lost to another former football player, Alonzo Highsmith . His record in boxing was 15 wins, 2 losses, and 1 no contest.

He admitted years later the real reason why he retired was fear of failing another drug test and the Jets learning he was using anabolic steroids. Convicted for drug possession in 1993, Gastineau has had several run-ins with the law, including domestic violence. His ex-wife, Lisa, and their daughter, Brittany, are the stars of the new E! reality television show, The Gastineau Girls.

Gastineau won the American Superstars competition in 1985.