Guillermo Apolinario Vilas (born 17 August 1952, in Buenos Aires, Argentina) is a retired professional tennis player from Argentina.
Raised in the seaside resort of Mar del Plata, Vilas was a southpaw and played his first tour event in 1969. He was in the year-ending top ten from 1974 through 1982. He was a clay-court specialist but also played well on hard, grass, and carpet surfaces.
He won four Grand Slam titles: the 1977 French Open and the 1977 US Open (both played on clay) and the 1978 and 1979 Australian Open (both played on grass). He was also the runner-up at the French Open three times (1975, 1978, and 1982) and at the Australian Open once (January 1977).