The British actors began dating after growing close on the set of the hit fantasy series, in which Harington's character Jon Snow falls in love with Leslie's Ygritte.
The couple split last August (14) after a year of dating, but sparked rumours of a reunion last week (ends25Jul14), when the pair arrived at Los Angeles International Airport together before heading to San Diego's Comic-Con event to attend a Game of Thrones presentation.
Now a source tells Us Weekly magazine Harington and Leslie are giving their relationship another chance and have actually been quietly dating for some time.
The insider says, "They've been back on for about three months. That said, it's casual right now and they're having fun. They're not moving in together or anything like that yet."
The couple's onscreen romance didn't end so well - Ygritte was killed off in season four.