Kylie Jenner Rings in 20th Birthday by Showing Off Her Bikini Bod, Celebrates with Sisters

Kylie Jenner's smokin' hot body is now 20 years in the making -- and she decided to show off the goods a day before the big 2-0 ... to all of our delight.

Kylie posted these shots of herself Wednesday -- the day before her 20th birthday -- proving she's all woman as she exits her teens. With curves like these ... her 20s are looking good.

Later in the day she celebrated her bday with her mom, most of her sisters (sans Kourtney), her friends and, of course, her man, Travis Scott.

They held a private shindig at the Hills Penthouse in West Hollywood ... where there was a chocolate fountain, a curvy ice sculpture (in honor of Kylie, obviously) and a cake with her face on it.

Kylie had every reason to celebrate and then some. As we reported, her mom says her lip line, Kylie Cosmetics, is now a $400 million company ... and on track to be a billion dollar company by 2022.

Congrats, kid! Ahem, we mean lady ... woman ... your grace.