The boy was born at 2:24 a.m. Sunday at St. John and St. Elizabeth Hospital in London and weighed in at 7 lbs. 7 oz., Stewart's publicist announced in a statement.
"Both Penny and our baby son are gorgeous, healthy and doing well," said Stewart, who was present for, and participated in, the natural birth. "I am so very proud of my girl Pen for the commitment, courage and strength she showed throughout."
Lancaster, a former model, added: "It means so much to Rod and I that we were able to keep to our birthing plan and have our baby naturally. It was the most empowering and spiritual experience of my life."
The pair, who have been together for nearly six years, are currently at home in Epping, England. "We are celebrating every moment with our beautiful boy, we just can't take our eyes off him," said Stewart.
Lancaster, 34, and Stewart, 60, previously said they're aiming for a spring 2006 wedding.