KJ Apais absolutely SICK of the rumors that he’s secretly Mr. Fantasy … so naturally, he responded by delivering the most dramatic crash-out imaginable.

Watch the video … the “Riverdale” star jumped online claiming the mysterious singer has completely destroyed his life — accusing him of stealing basically every ounce of his dignity … saying, “If anyone thinks it’s okay to take someone’s image, literal tattoos, and use it for their own success … it’s completely wrong.”

KJ says the situation’s gotten so bad he can’t even land “serious work” anymore because Hollywood now sees him as one giant walking meme instead of an actor.

“It’s hurting me and my career … I just lost out on a huge job because people think that I’m a joke because of this guy,” he said.

You’ll recall … we got Mr. Fantasy in Hollywood back in March and asked him straight up if he was KJ … and things got real weird. Mr. Fantasy’s true identity remains shrouded in mystery … of course, with lots of speculation he’s actually KJ.

He also accused Mr. Fantasy of using people close to him in a recent music video for the single, “Do Me Right” … and it’s chock-full of celebs … including KJ’s “Riverdale” costars Camila Mendes, Madelaine Petsch,and Lili Reinhart.

And then came the full meltdown — with KJ calling the artist a “f***ing idiot,” “liar,” and “thief” … basically sounding like a guy who just discovered his own burner account.