"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!" Deeley tweeted.
Deeley announced she was pregnant in early September.
The British television personality wed Kielty, a 44-year-old Irish comedian, in a hush-hush ceremony in Rome, Italy, in October 2012.
The couple first met in 2002 when they hosted the U.K.'s Fame Academy reality series together. Deeley and Kielty reportedly began dating in 2011.
So You Think You Can Dance has not officially been renewed for a thirteenth season, but Fox executives revealed recently the show is likely to return this summer.
"We hope it will return," Fox chairman Dana Walden told reporters at the Television Critics Association on Friday, according to Deadline Hollywood. "We're not finished in our conversations with the producers, but we love the show and hope and anticipate that it will be coming back."
Walden explained the reality dancing competition may return with format changes -- or at least some twists and surprises.
"We would see some changes -- how significant you perceive them to be is up to you," Walden noted. "It won't fundamentally change the show, it will just give it a little bit of spin."
SYTYCD creator, executive producer and judge Nigel Lythgoe also tweeted on New Year's Eve that fans of the show can "stop worrying."