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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. Getty

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

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