"We're expanding the brand," the actor told "Entertainment Tonight" Friday. "We're very excited."
The baby, which is due "early in the fall," will be the couple's second.
Weatherly's wife, internist Bojana Jankovic, gave birth to their first child, Olivia, one year ago, People magazine reported.
Weatherly has a another child, son August, with his first wife, actress Amelia Heinle.
He said the couple will find out the gender of the baby in advance, but did not commit to sharing that information, People reported.
Celebrity Couples of 2014 [PHOTOS]