It is officially ON between SHOOTER JENNINGS and JOHN MAYER, after Shooter BLASTED John on Twitter yesterday.

The origin of Shooter's rage is Mayer's recent "Rolling Stone" interview . . . which Shooter read in the bathroom.  Here's what Shooter had to say . . .

"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 RS article."

"Claims he's 'done w hollywood game' and has 'roots' and then immediately cuts hair, goes back to hollywood clubs + starts banging Katy Perry."

"And don't anybody say he's a great guitar player.  Gary Clark Jr & @lukasnelson wipes their asses with this dude."

"Sorry to [crap] talk. Lord knows not everybody likes my ass.  But seeing that RS in my friggin bathroom & reading the narcissistic drivel…"

No response yet from John Mayer., but people have been unloading on Shooter on Twitter!

Shooter is the son of country legend WAYLON JENNINGS.  And he's married to former "Sopranos" minx DREA DE MATTEO.