8:28 AM PT -- Well, that was quick ... Fury now says the retirement post was just a joke -- and he DOES plan on fighting again.
"Hahahaha u think you will get rid of the GYPSYKING that easy," Fury said ... "I'm here to stay."
He added, "Soon as I get better I'll be defending what's mine the heavyweight thrown."
It's over for Tyson Fury -- the heavyweight champ just announced his retirement in a blaze of anger ... referring to the sport as a "pile of s***t" and inviting his critics to "go suck a d**k."
As we previously reported, 28-year-old Fury pulled out of his rematch with Wladimir Klitschko after testing positive for cocaine on Sept. 22nd.
Fury's been taking all sorts of heat in the media -- and moments ago, lashed out.
"Boxing is the saddest thing I ever took part in, all a pile of s**t, I'm the greatest, & I'm also retired, so go suck a d**k, happy days."
Fury hasn't denied the cocaine allegations -- in fact, he changed his Twitter avatar to put his face on Tony Montana's body from that famous mound of coke scene from "Scarface."