Michael Phelps and his wife have welcomed a baby boy.

The couple - who already have 21-month-old son Boomer together - have added another child to their family with the birth of Beckett Richard Phelps on Monday (12.02.18).

Olympic gold medallist Michael shared a picture of his wife Nicole and their sons on Instagram and wrote: ''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. 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 (sic).''

Nicole posted her own picture and wrote: ''Our family grew by 1 yesterday, meet Beckett Richard Phelps. I'm surrounded by boys and I couldn't be happier (sic).''

And the couple even shared a picture on Boomer's official Instagram page, adding their own message from Beckett's big brother.

The caption said: ''I am officially a big bro!!! All I wanna do is hold him! I can't wait til I get to teach him so many cool things about the world!! #bigbro (sic).''

Michael and former Miss California USA Nicole, both 32, welcomed Boomer in May 2016 and tied the knot later that year.

And he previously revealed that the couple were hoping to expand their brood and that Nicole would also like a daughter in the future.

He said: ''Nicole has recently brought up that she would like a girl!

''We definitely want more kids, it just depends on when. I'm sure I'm getting pressure. [We're] trying to get moving on some things!

''We're going through life every day happy and blessed with what we have. But I hope to have more kids in the future.''