Both took to Instagram to post the happy news, showing photos of the actress wearing a pear-shaped diamond engagement ring and tagging each other in the pics.
A source confirmed the engagement to E! News.
The actress and the DNCE singer have been dating since at least November 2016, but have kept things very quiet about their high-profile relationship.
While the twosome have been spotted out and about on a slew of occasions, the duo has yet to do a red carpet together and are all but absent from each other's social media pages, save for today's big announcement (and one post from the GoT star last January).
But that's all changing now that Joe's put a ring on it...
Previously the singer dated Taylor Swift, AJ Michalka, Demi Lovato, Camilla Belle, Ashley Greene and Gigi Hadid.
Meanwhile, Sophie talked to Marie Claire UK in July about how happy she was with Joe, but said there were some issues being under the microscope.
"You do feel like you're living in a fishbowl," she said. "It's frustrating [that] it's the most mundane things that make the news—how boring!"
Today's news was anything but boring!
Take a look at the low-key couple's romance in photos...