The 31-year-old joined his US teammates in their first training session at the Olympic Aquatics Centre on Tuesday afternoon, just hours after arriving in the city for the August 5-21 Games, reports Xinhua.
After a short stint in the warm-up pool, Phelps moved to the main competition venue, starting with freestyle before switching to backstroke and breaststroke.
He dedicated just a few laps of the 50-minute session to butterfly, his only individual event in Rio.
Phelps is the most successful Olympian of all time, having won 22 medals - 18 of them gold.
In Rio, he will compete in the 100 metre and 200m butterfly and 200m individual medley. He is also expected to be a part of the US relay teams.
In addition to adding to his Olympic medal tally, Phelps is aiming to become only the third athlete to win gold in the same event at four consecutive Olympics, joining Al Oerter (discus) and Carl Lewis (long jump).
He could achieve the feat in both the 100m butterfly and 200m individual medley.