Steven Tyler Engaged?! -- Dude Looks Like a Fiance

Steven Tyler is domesticating himself ... because his longtime GF was sporting a huge rock on her finger this weekend ... and sources close to the rocker tell us it's an ENGAGEMENT RING!

63-year-old Tyler and 38-year-old Erin Brady hit Maui on Xmas day ... along with a massive new diamond. It's unclear if Tyler proposed before or during the tropical vacay.