Dutch swimmer Ferry Weertman won gold in the men's 10-kilometer open water race in the sewage-filled waters off Copacabana beach on Tuesday. Spyridon Giannitois of Greece earned silver and France Marc-Antoine Olivier took the bronze.
USA's Jordan Wilimovsky finished fifth and American Sean Ryan was disqaulfied.
Rio de Janeiro is only the second Olympics holding the competition in open ocean waters. At the first Summer Games in 1896, before custom-built pools were the norm, swimming was held in Greece's Bay of Zea.
The inaugural open water event in 2008 was staged at Beijing's rowing and canoeing canal and the open water swim in 2012 took place in Serpentine lake at London's Hyde Park.
On Monday, Brazil's Poliana Okimoto won bronze in the open water swim when France's Aurelie Muller was disqualified for colliding with another swimmer. It was the first Olympic swimming medal ever won by a Brazilian woman.
Sharon van Rouwendaal claimed gold for the Netherlands.