Deeley shared the news on Twitter with her fans, writing: "Beyond delighted to announce the birth of our baby boy last week.
"We couldn't be happier. Three really is the magic number!"
Deeley and Kielty married in 2012
The new mum received congratulations messages from her 360,000 Twitter followers, including Ant and Dec.
The TV presenting duo wrote: "Congratulations @catdeeley and @PatricKielty on your new arrival. Lots of love to the three of you."
Deeley replied: "Thank you darling! We are completely in love... Can't wait for you to meet him xxxxx"
Deeley revealed she was expecting her first child with Kielty via Twitter on 3 September.
She tweeted: "Exciting news... Paddy and I are so happy to be expecting our first child in the spring... Lucky us."
The pair first met when they presented talent show Fame Academy together in 2002, but didn't go public about their relationship until March 2012.
They married in September 2012.