"Both mom and baby are wonderful and healthy," said Dern's representative Cara Tripicchio, who added the new baby's name has not been disclosed.
Dern and Harper are also planning their wedding, but no date has been set, reported E! Online Monday.
It will be Dern's first marriage although she had been engaged to Jeff Goldblum and Billy Bob Thorton.
Dern's latest film "Happy Endings," will kick off the 2005 Sundance Film Festival. She also appears in the adaptation of the bestselling memoir "The Prize Winner of Defiance, Ohio" and then the movie version of Stephen King's "The Girl Who Loved Tom Gordon."