Michael Phelps became a father on Thursday when his fiancee Nicole Johnson gave birth to a boy, the Olympic swimming champion announced on Instagram.

Three months before the Rio Olympics, where he hopes to add to his record haul of 18 gold medals, Phelps posted a photo of himself cradling the baby.

"Welcome Boomer Robert Phelps into the world!!! Born 5-5-2016 at 7:21 pm !!! Healthy and happy!!! Best feeling I have ever felt in my life!!!," he wrote.

Phelps, 30, is training for the United States Olympic trials that will be held from June 26-July 3.

He retired after the 2012 London Olympics, but announced a comeback less than two years later.