Conor McGregor -- 'I Have Decided to Retire Young' ... Internet Loses Mind

Conor McGregor is either messing with his fans ... or just made one of the most shocking announcements of all time ... implying he's RETIRING.

McGregor went to Twitter and wrote, "I have decided to retire young. Thanks for the cheese. Catch ya's later."

We've reached out to Conor's camp -- so far, it's radio silence.

We've also spoken with a handful of plugged in UFC people -- and so far, no one seems to know if it's a joke or if Conor is seriously hanging it up.

Meantime, Conor is scheduled to fight Nate Diaz at UFC 200 on July 9th.