The now retired Olympic swimmer announced the news that his wife Nicole had given birth on Instagram.
'Magical moments yesterday... Nicole and I would like to introduce Beckett Richard Phelps to the world! We had a healthy baby boy and a healthy mama,' gushed Michael.
'I truly do feel like the happiest man in the world. Being able to build our family to now 4 (6 with doggies) is so incredible! #familyof4now.'
The 32-year-old uploaded an adorable photo alongside his heartfelt words.
In the picture, his wife is seen wearing a hospital gown while cuddling her two sons.
The duo's oldest child Boomer smiles sweetly and the camera and the family's newest arrival sleeps peacefully in a striped blanket.
Thrilled to hear that little Beckett had safely entered the world, many fans sent the couple a sweet message.
'Congrats Michael and Nicole. And of course Boomer. New title of big bro now,' said one.
Another added: 'Congratulations! I love the names of both your boys!'
A third individual couldn't resist making a joke about the boys' futures as athletes.
'Congratulations, may God continue to bless you and your growing family. Here is to lil swimmers in training,' they quipped.
Michael and Nicole welcomed their first child in May 2016. A year after Booomer's birth, the American opened up about his life as a parent.
'Nobody can really prepare you for it, you're going through it and you're learning as you go,' he told Today.
He went on to say how he admirers Nicole for taking motherhood in her stride.
Michael added: 'Watching her as a parent, watching her as a mom, she's the best. She's always reading things and just trying to make sure that his life is the best that we can make it.'