Katy Perry and Justin Trudeau are going strong ... because she just shared a new photo of her planting a kiss on on his neck.

The "Dark Horse" singer just posted a photo dump on Instagram showing her recent holiday adventures, and among the 20 photos is an image of her kissing Justin on a yacht in a tropical location.

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Justin's smiling and looking at the camera as Katy leans in and kisses him.

Katy's ex, Orlando Bloom, is also in the carousel ... they've been co-parenting since last year's split, and it looks like they spent time with their daughter over the holidays as a family.

As we reported ... Katy and Justin went IG official just last month ... so expect to see more of the former Canadian Prime Minister on Katy's page moving forward.