The ink is still wet on Tua Tagovailoa's rookie contract ... but the Miami Dolphins top pick is already in the process of giving back -- vowing to help the communities that raised him.

The 22-year-old ex-Alabama superstar just signed a 4-year, $30.3 MILLION deal with nearly $20 mil signing bonus ... and shared a snap of the moment he became a millionaire on Tuesday.

Tua -- who was raised in Hawaii, went on to win a championship with Alabama, and got drafted by the Dolphins -- says he's going to do what he can for all 3 areas that left an impact on his journey.