Swimmer Michael Phelps, the most successful Olympian in history, is set to end his retirement.
The 28-year-old American, who has won 22 Olympic medals, is expected to compete at a meet in Arizona at the end of April.
"He's just going to test the waters a little bit and see how it goes," said long-time coach Bob Bowman. "I wouldn't say it's a full-fledged comeback."
Phelps quit after winning his 18th Olympic gold at the 2012 London Games.
The Baltimore Bullet also has the record for most gold medals at a single Games, winning eight in Beijing in 2008 to eclipse the mark set by compatriot Mark Spitz in 1972.
Bowman said Phelps is entered in three events - the 100 butterfly and the 50m and 100m freestyle - during the three-day meet in Mesa that starts on 24 April, but could also compete in the 50m butterfly "just for fun".
Britain's Steve Parry, who finished third to Phelps in the 200m butterfly at the Athens Games in 2004, is surprised his former rival is making a comeback.
"I cannot believe he has made the decision to get back in the water," said Parry. "He has got nothing left to prove in the sport."
He added Phelps not only had to get himself back in shape physically, he also needed to recapture the hunger to win medals again.
But he told BBC World: "If anyone can do it, then Michael Phelps can."
Phelps has been training with Bowman's team at the North Baltimore Aquatic Club.
Bowman said Phelps is "pretty far" from being back in top form but is enough shape to "swim competitively".
"He's got back into good shape since September," said the coach. "He can give a good effort and certainly not be embarrassed."
He added: "He just kind of wants to see where he's at. It's more really for fun. It's been nice for me to see him swim just for the joy of it really.''
Rebecca Adlington, who won double gold at the 2008 Olympics, said it was "exciting" to hear that Phelps would be racing again.
"All eyes will be on him, so no pressure at all," added the Briton, who retired last year after winning two bronze medals in London.
After winning a further four gold medals in London, Phelps insisted his career was over.
"I have achieved what I wanted to achieve," he said. "If you can say that about your career, then it's time to move forward, time to move on to other things. I finished my career how I wanted to."
Retired US swimmer Dara Torres, who won 12 medals in five Olympics, was not surprised by news Phelps was returning to competitive swimming.
"You know I feel sorry for those swimmers who thought they had a better shot of winning a gold because Michael Phelps was retired," she said.
"He's an unbelievable athlete and it's great for the sport of swimming."
Last year, long-term coach Bob Bowman said Phelps was capable of adding to his stunning tally of medals at the 2016 Olympics in Rio de Janeiro.
Phelps returned to training last year and re-entered the United States drug-testing programme. He has now completed his six-month waiting period to be eligible for competition.
USA Swimming said Phelps is expected to join fellow Olympians Ryan Lochte and Katie Ledecky at the meet in Mesa.
Ian Thorpe, a long-time rival of Phelps, attempted a comeback ahead of the 2012 Olympics but failed to make the Australian team.