Adrian Peterson -- Huge Rapper's Got My Back

Adrian Peterson finally has a celebrity ally ... rap superstar Lil Boosie ... who hooked up with the embattled running back for a photo in Texas.

We're told the two ran into each other in Dallas earlier this week ... after Peterson returned from NY where he was attending appeal hearings in his effort to get back in the NFL.

Boosie -- who's a huge sports junkie -- is still a fan of the embattled running back ... and wants everyone to know he still sees "greatness" in Peterson.