Nicolás Alejandro Massú Fried (born October 10, 1979, in Viña del Mar, Chile), nicknamed Vampiro (Spanish: "vampire"), is a Chilean tennis player, a former world number nine in singles, and a two-time Olympic gold medalist.

Massú was introduced to tennis at age five by his Hungarian grandfather, Ladislao Fried. From age 12, he was trained at the Valle Dorado tennis academy, near Villa Alemana, by Leonardo Zuleta, with whom he perfected his forehand and double-handed backhand. He later trained at the Nick Bollettieri academy, in Florida, United States, alongside Marcelo Ríos, and later at the High Performance Center in Barcelona, Spain.

Massú became a professional tennis player in 1997. That year he won the prestigious juniors year-end Orange Bowl tournament. He also claimed the boys doubles competitions at Wimbledon (with Peru's Luis Horna) and the US Open (with countryman Fernando González), and was junior doubles world champion.