The 7 pound, 7 ounce (3.37 kilograms) tot was born at the St John and St Elizabeth Hospital in London at 2.24am. Both mother and child are said to be doing fine at home in Epping, England.
In a statement from the proud father, who was present at the birth, Stewart says, "Both Penny and our baby son are gorgeous, healthy and doing well. I am so very proud of my girl Pen for the commitment, courage and strength she showed throughout.
"We are celebrating every moment with our beautiful boy, we just can't take our eyes off him."
Lancaster adds, "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 baby is photographer/model Lancaster's first and 60-year-old Stewart's seventh.
Stewart's children include SARAH, 41, from a teenage romance with SUSANNAH BOFFEY; Kimberly, 26, and SEAN, 25, by first wife, Alana Stewart; RUBY, 18, with former girlfriend KELLY EMBERG; and RENEE, 13, and Liam, 11, from his marriage to Rachel Hunter.
Stewart and Lancaster plan to wed next summer (06) now his divorce from Hunter is complete.