The society gallery owner and her husband Giorgio Veroni welcomed the baby boy, who was 6lb 14oz and is known as "darling boy" for the moment, late on Sunday 26 October.
"I really feel very blessed and lucky to have my little boy," Tamara told HELLO!
"All the staff at the Portland were absolute stars and I had the most wonderful experience again."
Tamara, 44, revealed her pregnancy to HELLO! in August, telling the magazine: "When you've been lucky to have two gorgeous girls, I think you should just be grateful."
Tamara has a five-year-old daughter Vivi with 42-year-old Giorgio, and a 27-year-old daughter named Anouska.
The three of them and Giorgio spent Saturday having a "last lunch" as just the four of them, with Tamara posting a sweet snap of her family on Instagram
Tamara also revealed that she suffered two miscarriages before falling pregnant again – but she kept them quiet as her mother was ill with womb cancer at the time.
"I didn't make it into a big thing because of mummy's illness," she said.
"I am quite matter-of-fact about those things; what will be will be. I had my eggs looked at and the doctor told me if I really wanted to have a baby I needed to do it properly, but I'm just not one of those girls who could get it together with a chart and an ovulation stick."