"Beyond delighted to announce the birth of our baby boy last week," she wrote. "All doing well and we couldn't be happier... 3 really is the magic number!"
This marks the first child for Deeley and her husband, Northern Irish comedian Patrick Kielty, 44.
The couple wed in 2012 during a picturesque ceremony at St. Isidore's College chapel in Rome. Their nuptials came one decade after meeting on the BBC's Fame Academy, a musical competition show that aired in 2002 and 2003.
They first announced the pregnancy in September.
Congrats to the whole family!