In a statement released Tuesday by Hewitt's publicist, the couple said: "This is a wonderful time and we are looking forward to starting our family together."
The 34-year-old actress was previously engaged to Scottish actor Ross McCall.
Hallisay, also 34, was a recurring character on the first season of "The Client List." He became a regular this season.
He also appeared on the CW series "Privileged."
Us Weekly first reported the news.