TMZ has learned ... Welch -- who played spiky haired Mike Newton in the first three "Twilight" flicks -- filed a joint petition for divorce with his soon-to-be ex-wife Marissa Lefton.
The two were married back in 2008 ... and according to the docs, filed at L.A. County Superior Court, they separated in December 2011.
Should be an easy parting of ways ... 'cause according to the legal papers, they have no community property to split up ... and no kids either.
Fun Fact -- Marissa played 3-year-old Annie in "Father of the Bride" back in 1991 ... pretty cool, right?!