Mareen Louie Harper (born August 15, 1960), better known by her nickname Peanut, is a retired American female tennis player, born in San Francisco, California to Ron and Alice Louie. Her elder sister, Marcie, also played on the WTA Tour, while her sisters Cici and Marisa, and brother Ronnie, all played at the University of San Francisco. She, along with her siblings, trained on the tennis courts of Golden Gate Park. She was given the nickname "Peanut" by her dad because she was the youngest of the five children.

Louie Harper won 14 junior national titles and was the top ranked 16-and-under tennis player in the United States in 1976. She reached the finals of the junior Wimbledon in 1977 and the semifinals in 1978.

She made her professional debut in 1976, and that same year also played the US Open, where she lost in the first round. In her 19-year career, she won four singles title and reached her highest ranking of No. 19 in 1985. Her best results at the Grand Slams was the third round, which she achieved several times. Among her best wins were victories over Gabriela Sabatini and Zina Garrison. She retired in 1992.