Justin Trudeau's Halloween Costume Is Nod To New Girlfriend Katy Perry

Justin Trudeau's giving a nod to his new boo Katy Perry with his Halloween costume ... and he even got his son involved.

The former Canadian prime minister joined forces with his son for a joint costume this year ... Justin's in a shark suit and his son Hadrien is a bloodied surfer whose board is missing a chunk from a shark bite.

It's a pretty cool father-son costume ... even before considering the reference to KP.

Remember, when Katy performed the Super Bowl halftime show back in 2015, a couple backup dancers behind her went viral ... they were wearing similar shark costumes to what Justin's got here.

Justin doesn't mention Katy in his post about the costume, simply saying ... "Ready for Halloween with Hadrien: he’s the surfer, I’m the shark (to his left). We built the costume together — a little father-son Halloween teamwork."

But it's pretty clear Justin is giving a nod to Katy ... he mentioned he's "to his left" and the left shark was the one most folks remember from her Super Bowl performance.