Justin is finally responding to Aaron's gripes about not being paid homage by JB and modern-day child pop stars ... and Justin's throwing some serious shade towards AC in the process.

The Biebs shared his two cents Wednesday night, writing on his Twitter, "Aaron carter i had your album when i was little. And bumped the song Aaron's party. I was like 7. if you need a hype man I got you."

The hilarious beef started Tuesday, when Aaron responded to a random fan on Twitter, who had wondered why Carter never reached the same success as Justin.

Aaron went off on a mini-rant, suggesting he's basically the OG Biebs and blasting Justin and others for failing to give him props for paving the way.