Sources close to the singer-songwriter tell TMZ ... he'll join host Scott Rogowsky -- aka Host Malone, aka Quiz Khalifa, aka Trap Trebek -- Sunday night to turn the daily game show into a wonderland ... with a prize of $25,000.

We're told John's set to queue up a question or 2 for all the HQties out there ... much like Robert De Niro did recently. Other celebs like The Rock and Jimmy Kimmel have guest hosted as well. We've learned John will also belt out a tune during the show's broadcast.