[1] A five-time Olympian, Gianniotis has won a total of 15 medals in major international long-course competition, totalling 5 gold, 5 silver, and 5 bronze spanning the Olympics, the World Championships, the World Cup and the European Championships.

Initially, Gianniotis was given as the gold medalist as he appeared to cross the finish line first, but the Dutchman Ferry Weertman touched the timing pad before him.

[citation needed] Gianniotis won the silver medal at the age of 36, being the oldest competitor in the event.

Gianniotis also claimed gold at the 2013 World Aquatics Championships in Barcelona in the men's 10 km open water swimming.

At the 2012 Olympic Games in London, he was 4th at the 10 km marathon swim, missing the bronze medal by 5 seconds.