Sebastian Alexander Morgan, who will be called Sam, was born Friday at The Mount Sinai Medical Center in New York City. The baby weighed 6 pounds, 4 ounces, said the actress' publicist, Linda J. Stewart, in a statement Monday.
He is the first child for Fields, 37, and her fiance, Christopher Morgan, 30, a Broadway actor. Morgan was in the delivery room when the baby was born.
The couple decided to have a child last summer, Fields said in a statement.
"We were on the subway one day and Chris said, `I'm ready to start our family,"' she said. "It was beautiful and I started to weep."
"The Facts of Life" ran from 1979 to 1988.
Fields has an upcoming book titled, "When Bad Women Happen to Good Men: A Collection of Short Stories."