Hailey Bieber is making a splash to wrap up summer ... looking gorgeous as she models a plunging one-piece swimsuit on a diving board. Gallery Launch Gallery Backgrid

Check out the sizzling pics ... she turned the diving board at the Chateau Marmont in West Hollywood into her catwalk as she worked the swimsuit, hitting all the angles like the trained model she is. Backgrid

Hailey elongated her toned legs with a matching pair of heels ... and wore her brunette hair down and parted to the side for this stunning photoshoot.

It's unclear what the pics are for ... but we are certainly excited to see the finished product. Gallery Launch Gallery

Of course, HB's no stranger to modeling her assets -- earlier in July, she showed off her sun-kissed skin in a tiny yellow-and-w

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