The adorable couple wore all black and matching smiles in the photo.
Although the pair is just announcing the big news now, ET learned the couple has been engaged since December but kept the news private.
Tyler and Gardner welcomed a boy, Sailor Gene, back in February. This is the second child for the Leftovers actress, 38, who shares a 10-year-old son, Milo, with her ex-husband, Royston Langdon. Gardner also has an 8-year-old son, Grey, from a previous relationship.