The two stars cheesed it up Saturday night ... posing together for a photo taken by professional BTS party photog Tyrell Hampton at the Academy Museum Gala in L.A., where the stars were out in force and dressed to the nines amid Oscar season.

Both women looked stunning in their outfits, with Selena walking the red carpet solo in a suit ... and ditto for Hailey, who was in a cropped dress. Justin Bieber wasn't there.

It seems they crossed paths either inside the museum at an after-party, and apparently felt chummy enough for a joint snap. Selena's holding Hailey's leg and they're both leaned in close. Tyrell's caption ... "plot twist" -- obviously referencing Hailey's recent sit-down.

You'll recall ... Hailey dished about the fan-fueled notion that she'd stolen Justin away from Selena, a narrative that's been going on for years now -- which she shot down completely.

She said she respected Justin and Selena's relationship while it was going on, and didn't interfere at all.

Eventually, JB and SG fizzled out and he got with Hailey ... and she even said she thinks he needed closure from his history with Selena to formally move on.

Hailey also said she and Selena were on good terms, and had even spoken since she and Justin got together ... and eventually hitched. As for the toxic fans pitting the two ladies against each other, HB had some choice words for them too.

If her words weren't' convincing enough for folks, the proof is in the polaroid. All is well!

BTW ... we weren't kidding when we said this was a Hollywood who's-who. The heavyweights showed up for this thing -- including George Clooney, Julia Roberts, Joe Jonas and Sophie Turner, Emma Stone, Olivia Wilde, Kerry Washington, Jessica Chastain, Keke Palmer, Glenn Close, Tilda Swinton, Rebel Wilson, John Cho, Adrien Brody and more.