Now, it sounds like he's not so sure.
While saying he's never been happier with his life - and certainly doesn't miss the grind of what it took to become the most decorated Olympic athlete - Phelps left the door open to change his mind before the 2016 Rio Games.
"I don't know what's going to happen in the future," Phelps said Monday. "I don't know what's going to happen tomorrow."
In Barcelona for the world swimming championships, Phelps spoke to The Associated Press and other international media organizations in a series of one-on-one interviews set up by his sponsor, Speedo. When asked by the AP, yes or no, whether he'll compete at the next Olympics, Phelps now says he hasn't planned that far ahead in his life.
That's a striking change from his comments before and immediately after the London Games, when he insisted his retirement was set in stone and it had always been his goal to quit swimming before he tuned 30.
Phelps will be 31 at the time of the opening ceremony for the Rio Games.
"I don't know. We're in 2013," he said, while adding, "There's nothing in the works."
There's plenty of time for a comeback. Phelps would likely want to begin training before the end of the year, which would allow him to get into peak condition leading up to the next world championships in 2015, an important steppingstone for the Olympics.
Phelps certainly isn't training at the moment. He jammed the small toe of his right foot on the edge of a sofa while at home in Baltimore, and aggravated the injury when he played in a golf tournament at Lake Tahoe.
He's wearing a boot cast on his foot while in Barcelona.
After some sightseeing and promotional appearances, Phelps took in a second night of swimming at the Palau Sant Jordi before heading back to the U.S. on Tuesday. He's accompanied by his new girlfriend, Golf Channel reporter Win McMurry.
Phelps was still seething about the U.S. loss in the 400-meter freestyle relay on Sunday. The American were edged at the finish by the French, a repeat of last summer's Olympics when a team that included Phelps also settled for silver.
He said the Americans should have gone with a different lineup, using Jimmy Feigen in the leadoff spot and Nathan Adrian as the anchor. Instead, Adrian went first and Feigen took over for the finish despite being the least experienced member of the U.S. foursome.
"We should never lose that race," Phelps said.