13 August 2016 01:44 pm Views - 12046
The 31-year-old, who has won 22 Olympic gold medals, quit after London 2012 only to reverse that decision in 2014.
"I'm not going four more years and I'm standing by that," he insisted after finishing in a three-way tie for silver in the 100m butterfly final.
"I've been able to do everything I've ever put my mind to in the sport. I'm happy with how things finished."
However, team-mate Ryan Lochte thinks Phelps will be in Tokyo in 2020.
"I guarantee he will be there,'' Lochte told NBC. "I think so. I really think so. So Michael, I'll see you in Tokyo."
Phelps can add to his Olympic tally in the 4x100m medley relay at 03:04 BST on Sunday.
"I'm happy," added Phelps, who said he wanted to spend more time with newborn son Boomer and fiancee Nicole.
Phelps made his Olympic bow in 2000, winning his first gold in Athens in 2004. He has 27 Olympic medals in total, nine more than his closest rival. (BBC)