The duo worked together to locate the pooch belonging to Leos Carax, a director currently working with Driver, 35, on the musical "Annette" in Brussels.
Driver filmed a video pleading with fans to help him find Carax's dog, Javelot. Carax, 58, owned the grandfather and father of Javelot, according to Driver.
"We will put you in the movie," Driver said. "We will give you chocolate, we'll christen your child. We'll do anything as a way of saying thank you."
Driver isn't on social media, but Hamill, 68, is -- and he's got well over 3 million followers.
Hamill posted the video, which got 33,000 likes and nearly 10,000 retweets.
And here's the best part: it worked.
According to The Hollywood Reporter, Javelot was seen at Carax's side at a train station after the video went viral.
Driver and Carax's film, also starring Marion Cotillard and Simon Helberg, will focus on a stand-up comedian and his opera singer wife. No release date has been announced.