Instagram/@katyperry

Justin Trudeau just leveled up ... he’s now officially rolling in Katy Perry ’s inner circle ... recently spotted partying with his GF and her crew as she blew out her birthday candles.

Katy dropped a flood of snaps from her 41st on Tuesday ... and in one candle-blowing clip , you can clearly spot Justin in the background, clapping and cheering like part of the fam. Instagram/@katyperry

It's just a quick glimpse of the former Canadian PM ... but with that grin, it’s clear he’s swapped politics for full-on groupie life.

As you know, K & J just went public in Paris last week -- only a few weeks after TMZ dropped those steamy yacht vacay shots ... and months after the first romance buzz back in July.

As for Katy’s birthday IG dump, it was pure

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