Sylvia Eliina Ruuska (born July 4, 1942) is an American former competition swimmer, Olympic medalist, and former world record-holder in two events.
At the age of 14, Ruuska won two medals at the 1956 Summer Olympics in Melbourne, Australia. She captured a silver medal by swimming for the second-place U.S. team in the women's 4×100-meter freestyle relay. Individually, she also received a bronze medal for her third-place finish in the women's 400-meter freestyle.
On June 27, 1958, she became the first woman to set an official world record in the 400-meter individual medley, clocking 5:46.6. Ruuska subsequently broke her own 400-meter record three times in 1958 and 1959, ultimately lowering the world mark to 5:40.2 on July 17, 1959. She also set a new world record of 2:43.2 in the 200-meter individual medley on August 16, 1958, and breaking her own record with a time of 2:40.3 on January 14, 1959. Both her 200-meter and 400-meter individual medley world records were later broken by Donna de Varona. At the 1959 Pan American Games in Chicago, she won a silver medal for her second-place showing in the 400-meter freestyle (5:03.4), finishing behind fellow American Chris von Saltza (4:55.9).
At the 1960 Summer Olympics in Rome, Italy, Ruuska swam for the gold medal-winning U.S. team in the preliminary heats of the women's 4×100-meter freestyle relay. Under the international swimming rules in effect in 1960, she did not receive a medal because she did not swim in the event final.
Ruuska was inducted into the International Swimming Hall of Fame in 1976.