The baby, named Dylan Christopher, arrived at 7:20 p.m. at Southern California's Tarzana Hospital. He weighed 8 lbs. 4 oz., and measured 21 inches, according to Gold's spokesman.
"Mom and baby are doing fine," her rep (and proud grandpa) Harry Gold tells PEOPLE.
Gold's pregnancy was announced in October. The actress, 38, and Marshall, 42, are also parents of sons Sage, 11, Bailey, 8, and Aiden Michael, 3.
The couple met in 1990, when Marshall was a consultant on the TV movie Blind Faith, which starred Gold's Growing Pains mom, Joanna Kerns. They married in 1994.
Growing Pains, on which Gold played Carol Seaver, ran from 1985 to 1992.