According to the legal docs filed in L.A. County Superior Court, "The parties intend to get married, so they must dissolve their registered domestic partnership."
Leigh's domestic partner -- a writer named John Morton -- filed the papers. They registered as domestic partners in 2008.
Leigh -- who also starred in "Peyton Place" -- is 66. She was formerly married to Ryan O'Neal and movie mogul Guy McElwaine.
So far, no word on when the couple plans to tie the knot.