"Congratulations to my beautiful and very tough wife @mollymcnearney on delivering our baby girl this morning," the 46-year-old Kimmel tweeted.
"Kids, don't do drugs. Until you have a baby. Then do ALL OF THEM," McNearney quipped soon after her husband made the announcement.
The new parents named the little one Jane, Kimmel's rep told The Times, adding that McNearney was "doing well." The moniker is pretty traditional and comes as a departure from Kimmel joking that he would have loved to name his next offspring "Karate" or "The Riddler." (We were totally rooting for Karate Kimmel!)
Jane is kid No. 3 for Kimmel and his first child with McNearney, a writer on his show whom he married in July 2013.
The comedian already has two adult children, daughter Katie, 22, and son Kevin, 20, from his 14-year marriage to Gina Kimmel, which ended in 2003.
The funnyman, whose recurring "Mean Tweets" segment has celebrities reading hostile online comments about themselves, was surprised by the warm sentiments he received after announcing his daughter's birth.
"Thanks for all your kind baby wishes - turns out, not ALL tweets are mean!" he wrote.
The host also received another surprise Thursday: His late-night talker was nominated for an Emmy Award for variety series.
Kimmel originally broke the baby news on "The Ellen DeGeneres Show" in February and highlighted that it had been a long time since he had to deal with a newborn.
"I've really forgotten everything," he said. "It's funny because I was with [friends John Krasinski and Emily Blunt], who just had a baby and I was like, 'Oh, yeah. And there's new ways to do things.'"