Kylie & Tyga ... Our First Public Kiss!!! (PHOTO)

Kylie Jenner and Tyga are showing PDA for the first time ... locking lips.

Kylie posted the pic to her Snapchat Friday night during Kris Jenner's 60th bday party. It's not a full make-out session ... but more than they've ever publicly displayed.

Buy hey, she only turned 18 in August.