The couple met while working on the show and began dating in 2011.
Goodwin was previously engaged to actor Joey Kern, while Dallas was married to Lara Pulver from 2007-2011.
On her official Twitter account, Goodwin wrote Friday, "From my soon-2-b-Mrs-Dallas head 2 my soon-2-be Mrs-Dallas-toes, I'm oh-so grateful 4 your well wishes! THANK U!"
Dallas tweeted to on his account, "Thanks to all of you for the outpouring of love and congrats!"
"Once Upon a Time" is now in its third season.