This is the couple's second daughter, People magazine reported Saturday.
"It's a girl!!! And she's as beautiful as her mommy and big sister Mabel," Lowe, 44, wrote Thursday. "We are blessed!"
The name of the new baby was not reported.
"I'm trying to bank some sleeping hours, which is a little tough," the "Pretty Little Liars" actor told People a week ago, while preparing for the birth.