Ariana Grande Locks Lips with Another Rapper

Ariana Grande and Mac Miller are going very public with their relationship -- sucking down sushi and each other's faces, and we got the pics.

The couple hit up Katsu-Ya in the Valley Sunday night and couldn't keep their hands off of each other -- kissing and giggling through the meal. Witnesses tell us at one point Ariana was even lying on his lap.

Sources close to them tell us they are dating, even though Mac's downplayed their relationship in interviews ... referring to Ariana as just a "homey."

They've got some history. Before Ariana hooked up with Big Sean ... Mac was on the scene as her boyfriend in her "The Way" music video.

Way to go, homey.