Olympic swimming gold medallist Mack Horton has thanked an eagle-eyed fan who noticed a suspicious mole on his chest and contacted the Australian team.

Horton, who won gold in the 400m freestyle in Rio, posted a message of thanks to his anonymous saviour on his Instagram account after having the mole cut out of his chest.

"Shout out to the person that emailed the swim team doctor and told me to get my mole checked out. Good call. Very good call," Horton wrote, bearing a bandage and the 'thumbs up' in the photograph.

The mole was removed on Thursday in Melbourne.