Ain't nuthin' but a g-string baby ... and Johnny Manziel loves it!!
The QB hit the beach in Miami for a little fun in the sun with his fiancee, Bre Tiesi -- who turned heads in a tiny electric green bikini ... and it was gooood.
Manziel has been training his face off for his football comeback -- recently doing sand drills with his former Cleveland Browns teammate, Joe Haden.
Of course, Bre looks amazing ... but Johnny looks great, too -- putting back some of the muscle he had lost when he was partying his way out of the NFL.
Looks like he's taking his comeback seriously -- but seriously, Bre Tiesi for the win.