John Mayer is a douchebag ... in fact, he's "King Douche" ... this according to country music bigshot Shooter Jennings -- the son of country legend Waylon Jennings.
Shooter -- a powerful country music DJ -- just went after Mayer on Twitter ... saying, "I love how the 'neil young of our generation' self-naming king douche grows his hair out + buys a place in Montana just to do [Rolling Stone] article."
Shooter continues, "Claims he's 'done w hollywood game' and has 'roots' and then immediately cuts hair, goes back to hollywood clubs + starts banging Katy Perry."
The attack continues ... "And don't anybody say he's a great guitar player. Gary Clark Jr & [Lukas Nelson] wipes their asses with this dude."
It's unclear if Shooter has any personal beef with Mayer ... or if he just felt like unloading for the hell of it.