Olivér Halassy[a] (né Haltmayer; 31 July 1909 – 10 September 1946) was a Hungarian water polo player and freestyle swimmer who competed at the 1928, 1932 and 1936 Summer Olympics.

He was a member of the Hungarian water polo teams that won one silver and two gold medals in 1928, 1932 and 1936.

He also became European champion in 1500 metres freestyle swimming in 1931, a few hours after he helped his water polo team to victory.

He was killed by a Soviet soldier near his home in Budapest, in what Hungarian sources called "tragic circumstances.

"[4][5] Other accounts indicate that he was coming home by taxi late at night when he was murdered by occupying Soviet soldiers in a robbery or mugging.