Check out Robert Griffin III and his wife Grete Sadeiko taking in some sun and sand in Miami Beach on Thursday ... showing off their amazing physiques.

Of course, Grete is a world class track athlete who's training for the 2020 Olympics -- so, staying fit is her job.

As we previously reported, Grete competed at Florida State where she specialized in the heptathlon -- once running 13.92 in the 100-meter hurdles. She's hoping to compete for her home country Estonia in the 2020 Games.

Meanwhile, RG3 is sitting comfortably as the 3rd string QB for the Ravens -- behind Joe Flacco and Lamar Jackson. The team is 4-5 and still gunning for a playoff spot.

The good news for Robert, he did well in the preseason and there's a chance he could get a shot somewhere else next season.

But, don't feel too bad, RG3's making $1,000,000 this season for being the backup -- not too shabby for a guy who hasn't taken 1 regular season snap since 2016.