While Tom Brady was throwin' bombs to his new teammate Antonio Brown ... Rob Gronkowski was busy ass-shakin' to some Backstreet Boys in an epic dance-off with his GF!!
The retired Patriots superstar is staying active on his NFL Sundays ... teaming up with his bikini model lady Camille Kostek for a public workout at the Boston Seaport with his family's Gronk Fitness business.
The workouts are great and all ... but we're 100% here for the hot couple getting down and GETTING IT to BSB's "Everybody (Backstreet's Back)" .