The So You Think You Can Dance? presenter and comedian husband Patrick Kielty quietly welcomed their first child together last week, but only made the happy news public on Tuesday evening (19 January).
A thrilled Cat tweeted: "Beyond delighted to announce the birth of our baby boy last week. All doing well & we couldn't be happier... 3 really is the magic number!"
She didn't reveal any other details such as the tot's name.
Cat and Pat announced they were expecting back in September, with Cat tweeting: "Exciting news.... Paddy and I are so happy to be expecting our first child in the Spring..... Lucky us."
West Bromwich-born Cat, 38, and Northern Irish comedian Patrick, 44, were friends for a decade before romance blossomed, having hosted the TV talent show Fame Academy together back in 2002.
Cat, who relocated to the US where she has found success as the host of primetime show So You Think You Can Dance?, confirmed their relationship in early 2012, telling a US TV show that she was "dating a really old friend of mine".
They wed in Rome in 2012.
Congrats to Cat and Pat!