Wong Maye-E, File, Associated Press
FILE - In this July 29, 2011 file photo, U.S. swimmer Michael Phelps talks to his girlfriend Nicole Johnson, right, after the medal ceremony for the men's 4x200 Freestyle Relay, at the FINA Swimming World Championships in Shanghai, China. The 18-time Olympic gold medalist and fiancÉ Nicole Johnson welcomed Boomer Robert Phelps on Thursday night, May 5, 2016, in Arizona, where Phelps has been training in preparation for the upcoming Rio Olympics.

Michael Phelps is a first-time father.

The 18-time Olympic gold medalist and fiancé Nicole Johnson welcomed Boomer Robert Phelps on Thursday night in Arizona, where Phelps has been training in preparation for the upcoming Rio Olympics.

"Healthy and happy!!! Best feeling I have ever felt in my life!!!" Phelps posted on Instagram Saturday in announcing his son's name and arrival date and time. A black-and-white photo showed a shirtless Phelps cradling his son and planting a kiss on his head while Johnson looked on from her hospital bed.

Phelps also had been training at high altitude in Colorado and left the Olympic Training Center in Colorado Springs to return to Johnson's side.

Last month at a meet in Mesa, Arizona, Phelps said the couple had already selected the baby's name, although he didn't reveal it then.

"It's something we both agreed on, thought it was different," he said.

Asked what type of parent he would be, Phelps said, "I'll probably be very protective. That's how I am."

Phelps credited Johnson for being "nothing but one of the best supporters I could ask for."

At the Mesa meet, Phelps talked with Dana Vollmer, who is attempting to make another Olympic team after giving birth to her son 13 months ago.

"It will be good for him, like it is for me," she said. "Michael and Nicole will be good parents."