Michael Phelps competed in five Olympic Games: Sydney 2000, Athens 2004, Beijing 2008, London 2012, and Rio 2016.
His Olympic career began when he was just 15 years old in Sydney, where he finished fifth in the 200-meter butterfly. He won his first Olympic gold medals in Athens and became the dominant swimmer of the Beijing Games, winning eight gold medals.
Phelps briefly retired after London 2012 but returned to competition, qualifying for the 2016 U.S. Olympic team. Rio was his fifth Olympics and his final Games. He concluded his career with 28 Olympic medals, including a record 23 golds.
A common mix-up is confusing Olympic events with Olympic appearances: Phelps entered many races within each Games, but the question asks how many separate Olympic Games he attended.