Ariana Grande & Nathan Sykes -- Yes, We're Dating ... Sorry Justin Bieber

Almost is never enough for Ariana Grande and The Wanted singer Nathan Sykes ... so they're going all the way ... sources tell TMZ, the two are OFFICIALLY dating.

Ariana and Nathan have been canoodling for weeks -- and even filmed a steamy music video together for their song "Almost Is Never Enough" -- but according to sources close to the couple, they've now officially DTR'd.

This means Justin Bieber -- who was rumored to be hooking up with Ariana -- is completely out of the picture.