Actress Jessica Chastain has turned pet detective following the alleged theft of her grandmother's dog.

The Help star has taken to her Instagram account to urge followers to help her track down the missing Maltese terrier after little Livvy was snatched outside a McDonald's fast food restaurant in Vallejo, California.

The Crimson Peak star is offering a reward for the pet's safe return.

Posting a photo of Livvy online, Jessica writes, "My grandma's dog, Livvy, was stolen last night from McDonald's at 2565 Springs Rd Vallejo, CA. Please forward this to anyone you know in CA (California). There's a REWARD for Livvys (sic) safe return.

"For those of you who love animals, you must understand how devastated my grandma is. Please help."