The American couple, who originally announced that they were expecting back in November 2015, couldn't wait to share the sweet news on their social media accounts, and even treated fans to their baby name choice.
Michael, 30, took to Instagram to share a stunning black and white snap of his new family, captioning the photo: '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!!! @nicole.m.johnson and Boomer both healthy!!! #boomerphelps'
Boomer is certainly an unusual choice, which Michael had previously hinted at during a meet at Mesa, Arizona, where the sports star has been training ahead of the Rio Olympics. 'It's something we both agreed on, thought it was different,' he said.
When asked what sort of parent he thought he'd be, the star said: 'I'll probably be very protective. That's how I am.'
Fans were quick to congratulate the pair on their arrival, with one commenting: 'that baby will be sooo loved. not just by his mommy & daddy, but by his fans. congrats', while another added: 'Congratulations on the new little addition to your family. Babies change and grow so quickly. Be sure to capture everyday moments and milestones!'
Michael and his fiancée, who've been dating on-and-off since 2009 (they officially split late in 2011 before rekindling their romance in 2014), have said they will get married after the Olympics is over on 21st August.
The pair had already revealed they were expecting a baby boy, which 22-time medallist Michael couldn't wait to share during their original pregnancy announcement.
They opted for a very fitting photo, showing the couple holding a pair of tiny swimming trunks with the initials 'MP' on them. The caption read: 'We found out 8weeks ago we were expecting... Today we're over 12 weeks and it looks like we're having a boy!!! #babymp #mp #lifeabouttochange #mpswim 📷 @djnolan5'
Congratulations to the new family of three!