Larry Fitzgerald could end up in a pool of slime ... or something WORSE ... when he battles Seattle Seahawks star Earl Thomas in a loser-gets-dunk-tanked competition at the Nickelodeon Kids' Choice Sports Awards ... TMZ Sports has learned.
We're told ... Larry and Earl have both signed up for something called the "Verizon Power Dunk" -- in which they will each sit in a dunk tank and try to hit a bull's-eye that will sink the other.
The catch -- the dunk tanks will be filled with a "gooey" mystery substance ... and we're told It could be slime or something worse.
We're told the fans will ultimately decide what goes in the tank via a live vote when the award show goes down on Thursday, July 17th at 8pm.
Don't worry fellas ... if Bieber can take a sliming, you guys should be just fine.