Stephen K. Lundquist (born February 20, 1961) is an American former competition swimmer who is an Olympic gold medalist and former world record-holder. At the 1984 Summer Olympics in Los Angeles, he won gold medals in the 100-meter breaststroke and the 400-meter medley relay.

Lundquist was the first swimmer to break two minutes in the 200-yard breaststroke, and won every 100-yard breaststroke event he entered from 1980 to 1983. At age 17 he broke his first world record and in his career he set new world and American records on 15 occasions. He first broke the 100-meter breaststroke world record in 1982 and held it until 1989, with the exception of one month in 1984 when John Moffet broke it in June at the U.S. Olympic Trials (with Lundquist reclaiming it at the Olympic Games in July). He also held the world record in the 200-meter individual medley in 1978. He set American records in the 100-meter and 200-meter breaststroke and the 200-meter individual medley.

Coached by Arthur Winters, Lundquist switched from a butterfly swimmer when he was 12 years old to the breaststroke, which is the stroke he came to dominate. Winters was at the end of the pool when he broke his first world record at 17 years of age.

Lundquist went on after the 1984 Olympics to spend much of his time volunteering his time for charitable organizations and making appearances on television and in movies. In June 1985, People Magazine named him as having the Best Chest of male celebrities, including a full-page picture of his muscular torso. In 1996 when the Olympics were hosted in Atlanta, Georgia, he was an Olympic torchbearer, the Clayton County Master of Ceremonies for the torch run, and the Olympic flagbearer at the 1996 Olympic Games.