Alfonso, who is famed for playing Carlton Banks in 90s sitcom Fresh Prince of Bel-Air, announced the happy news after performing on US talent show Dancing With The Stars.
Alfonso and Angela welcomed their first child together, son Alfonso Lincoln Jr., exactly a year ago today.
After performing a rumba on the dance floor, 43-year-old Alfonso gushed: "My son, it's his first birthday today, so I'm so happy that he's here with me. And as a wonderful present, my wife and I decided to get him a little brother or sister.
"We're having a baby! I wanted to announce it here on the show because I just wanted to say thank you to all the fans and say I love my wife."
This will be Alfonso's third child, as he is also dad to daughter Sienna from a previous relationship.
Alfonso tied the knot with Angela at a sunset ceremony in California in October 2012.
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