'Twilight' Star Michael Welch -- DIVORCE ... On Valentine's Day

"Twilight" star Michael Welch spent his Valentine's Day doing something special with his estranged wife -- THEY FINALLY FILED FOR DIVORCE .... how romantic!

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.