After delivering a dance that got him the second-highest scores of the evening, the 43-year-old Fresh Prince of Bel-Air star took the opportunity to wish his infant son a happy birthday, and then he revealed the big news.
"My son, it's his first birthday today, so I'm so happy that he's here with me," Alfonso said, smiling from ear to ear. "And as a wonderful present, my wife and I decided to get him a little brother or sister."
Alfonso's pro partner Witney Carson had clearly been let in on the good news, but it came as a surprise to DWTS co-host Erin Andrews and the audience who cheered in support.
"We're having a baby!" the actor said excitedly. "I wanted to announce it here on the show because I just wanted to say thank you to all the fans."
Alfonso also has a 12-year-old daughter named Sienna from his first marriage to actress Robin Stapler.
To hear what Alfonso had to say about his big news backstage, check out ET's interview below.