Remarkably, her pregnancy never became public.
Linney was last photographed at a Hollywood event in June with no visible baby bump.
In September, she took home an Emmy award for best actress in a miniseries or movie for Showtime's "The Big C: Hereafter," but she was a no show at the ceremony.
This is the award-winning actress' first child with her husband of three years.
Linney's representative says mother and baby are very happy and healthy.