The talk show host, 46, is expecting a baby with wife Molly McNearney, 35, a co-head writer on Jimmy Kimmel Live, he announced in an appearance on The Ellen DeGeneres Show airing Wednesday.
It's the first child together for the couple, who started dating in 2009, got engaged while on safari in Africa in 2012, and married last July.
Kimmel has two children – son Kevin, 20, and daughter Katie, 22 – from his previous marriage.
"I've really forgotten everything. It's funny because I was with [friends and neighbors John Krasinskiand Emily Blunt], who just had a baby and I was like, 'Oh yeah.' And there's new ways to do things," he jokes to Ellen DeGeneres.
"I told my daughter, 'Listen, there's only so much love I have to give — you're gonna have to split it up with the new kid. There's a finite amount of love and that's that.' So they're dealing with it, but it is very weird at my age. I'm going to be one of those old dads."