He was named the Greek Male Athlete of the Year, for the years 2011, 2012, and 2013. He won the silver medal at the 2016 Olympic Games in Rio at the 10 km marathon swim. 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. After 15 minutes of deliberations by the judges and the use of photo finish, Weertman was awarded the gold medal and Gianniotis the silver, despite the fact that both were credited with the same time: 1 hour, 52 minutes, 59.8 seconds (1:52:59.8). Gianniotis won the silver medal at the age of 36, being the oldest competitor in the event.