Goodwin's spokesperson confirmed the news to OTRC.com Wednesday morning.
Goodwin and Dallas, who play Snow White and Prince Charming on ABC drama, announced their engagement in October. The couple went public with their relationship last year.
It is unclear how Goodwin's pregnancy will affect production on "Once Upon A Time."
Goodwin was previously engaged to "Super Troopers" actor Joey Kern. It was reported in May 2011 that they had broken up.
Dallas was previously married to British actress Lara Pulver, known for playing Irene Adler in the BBC series "Sherlock" and Claudine Crane in HBO's "True Blood," for four years.