Spelling, 39, and McDermott, 45, are already parents to Liam, 5, Stella, 4, and Hattie, 10 months. The couple have named the new born Finn Davey McDermott, who weighs 6 pounds, 6 ounces, reported People magazine online.
"It's a boy!!!!," Spelling posted on Twitter.
The former 'Beverly Hills, 90210' star also wrote, "We are so excited to announce the birth of the newest member of the McDermott family."
She posted a picture of a little hand, presumably that of her newborn son.
Spelling announced her pregnancy in March, just five months after giving birth to Hattie. She hinted earlier this week that it would be her last pregnancy.
"I couldn't be happier that I'm finally at the end and I get to meet the baby so soon, but I actually love being pregnant, so if it is my last, I'll be sad never to be pregnant again," she said.
