Hailey Bieber Shows Off Some Butt Cheek Amid Summer Temps in L.A.

Summer isn't technically here yet ... but for Hailey Bieber, it's close enough to show off some skin!

The model and makeup mogul posted a thirst trap to Instagram showing off some butt cheek in a bikini ... declaring in the caption, "it's summer."

We'll let it go that it's not summer just yet, despite how hot it's been in the City of Angels ... but it's a little confusing as to why she also included a snap of her watching Snoopy from "Peanuts." Hey, whatever!

Hailey also dropped some thirst traps a few days ago ... including one of her face and upper body in a cute, red & black polka dot top, captioning the post, "clean the front camera. xx."