The "Sisterhood of the Traveling Pants" star, 28, and Cross, 47, have been together quite a while -- they were rumored to be dating in April 2008, but didn't officially step out together until 2009.
Tamblyn was most recently seen on the big screen in "127 Hours" and on the FOX TV show, "House." Cross meanwhile, has been dodging rumors of an "Arrested Development" movie, in which he would revive his role as the quirky and infamous never-nude Tobias Fünke.
Tamblyn isn't the only "Traveling Pants" star to take the marriage plunge. She was recently spotted at the wedding of her co-star America Ferrera in upstate New York in June.
Watch Cross' former co-star Jeffrey Tambor discuss the possible "Arrested Developement" movie below.