Greta Andersen (born 1 May 1927) is a retired Danish swimmer who won a gold and a silver medal in 100 m freestyle events at the 1948 Olympics. In the mid-1950s she moved to the United States, where she set several world records in marathon swimming in the distances up to 50 miles.
Andersen began swimming in a club aged 16 while also doing gymnastics. In 1947, she won two European medals at her first international competition. Next year she won two more medals at the London Olympics, a gold in the 100 m freestyle and a silver in the 4×100 m freestyle relay. She failed to finish her 400 m freestyle race due to sudden stomach cramps – she fainted and was rescued from drowning by fellow competitors Nancy Lees and Elemér Szathmáry. In 1949 Andersen set a world record in the 100 yard freestyle at 58.2 s, which stood for seven years. She took part in three events at the 1952 Olympics, but could not use one leg due to a recent knee surgery, and failed to medal. Her best result at those Games was a fourth place in the 4×100 m freestyle relay. During her career in Europe, she won nine individual Danish titles, several team titles, and four individual Scandinavian titles.
In the mid-1950s she immigrated to Long Beach, California, and later obtained US citizenship. There she switched to marathon swimming and became the first person to swim a major channel both ways (Santa Catalina Channel in 1958). She also set world records in the 10, 25 and 50 miles. Between 1957 and 1965 she crossed the English Channel six times, setting a record for most Channel swims by a woman, as well as a speed record for women at 10:59 h in 1958. She also set an unofficial record for the longest Channel swim, while trying for 23 h to cross the Channel forth and back in 1964.