The pair met on the set of the ABC drama, where Morrison plays Emma Swan and Stan stars as The Hatter.
A source told People that "they started dating three months ago. They seem to be really happy together, but are trying to take things slow."
The pair aren't the first couple to have been brought together by the show - Ginnifer Goodwin and Josh Dallas, who play Snow White and Prince Charming, made their relationship public in April.
Once Upon a Time's new season begins in September, with new fairytale characters such as Aladdin and the Little Mermaid set to be introduced to the current cast.
Continuing the theme of characters made famous in animated Disney adaptations, The Hangover's Jamie Chung has also been added for the second season cast as the Asian warrior Mulan, alongside The Tudors star Sarah Bolger as Sleeping Beauty.
Watch the Once Upon a Time writers discuss the show below: