The 'Guardians of the Galaxy' actor popped the question to his longterm partner Natasha Halevi while on a romantic vacation to Kyoto, Japan.
Sean, 44, proudly announced the news on Instagram: "I asked @tashalitas to marry me and she said yes. Or to be more precise she said, 'a million times.'"
The former 'Gilmore Girls' star posted an adorable selfie of himself and Natasha, along with a picture of their entwined hands, showing off her huge diamond ring.
Natasha also took to social media to share the happy news, posting a shot of them both sharing a passionate embrace.
She captioned the photo: "Wrote a long thing, but, instead. . . WE'RE ENGAGED YOU GUYS! (sic)"
Sean is best known for playing quirky neighbour Kirk Gleason in the long-running hit series 'Gilmore Girls'.
He also plays space pirate Kraglin in the 'Guardians of the Galaxy' films, both of which were directed by his brother, James Gunn.
Director James was one of the first to congratulate the happy couple.
Sharing a cute photo of Sean and Natasha with some penguins, he wrote: "Huge congrats to my brother @thejudgegunn & @tashalitas on their engagement!!!! Looking forward to many more wonderful excursions together!! (sic)"
While in Japan, Sean has been mixing business with pleasure, appearing at Tokyo's HollyCon to pose for pictures with 'Guardians of the Galaxy' fans.
But he and Natasha have also been enjoying a romantic trip together, with Sean writing before the proposal: "Kyoto is even more magic with someone you love."
Natasha is a comedy actress whose biggest role to date was as one of he lead ensemble cast members in 2015 TV series 'Lunch Break Feminist Club'.