Ryan Lochte -- I'm Engaged To My Playboy Model GF!!

Ryan Lochte is gettin' hitched to his Playboy model girlfriend Kayla Rae Reid ... poppin' the question to the smokin' hot blonde tonight in Malibu.

Lochte -- who's currently on "Dancing with the Stars" -- proposed to Reid with a HUGE ROCK in the Malibu Canyon at sunset.

Ryan hit up Instagram to announce the good news ... saying "Memories forever!!! #thelochtes #LA."

FYI -- Reid and Lochte have been dating since March ... and started to get serious in April when R.L. introduced the model to his parents.

Well ... at least he has time to plan the wedding!