Jake Paul Pranks Fiancée Jutta Leerdam During Bikini Shower

Jake Paul pulled a fast one on his new fiancée ... hitting her with a classic shower prank as she was cleaning up in a bikini -- and he earned a W with the execution!!

El Gallo shared the hilarious footage on Tuesday ... showing Jutta Leerdam rinsing off under the blue sky -- but little did she know the boxer had some tomfoolery on the itinerary.

As the Dutch speed skater let the water spill over her noggin, Jake had bottles of his body care line on deck ... and started dousing her in his product as she tried to get the suds off her.

But as she got close to being done, Jake continued to pour his soap all over Jutta's athletic figure ... and naturally, she got super confused and let out a "What is this?!?!"

Eventually, Jutta cleared her eyes of bubbles and tracked down the culprit ... and asked, "What the f***?!?"

Chances are Jutta was in on the bit ... but it's a solid marketing tactic nonetheless.