The singer hit the town Saturday night with Ethan Slater -- who stars opposite her in the upcoming 'Wicked' movie ... and whose big claim to fame thus far is playing the famed SpongeBob SquarePants live on stage (the theatre version, not from the TV show).

Eyewitnesses tell TMZ the two love birds appeared to be having a bite -- or drinks, more like -- at the MO Lounge within the luxurious Mandarin Oriental hotel in the heart of Manhattan.

As you can see in these photos we obtained ... they were off in the corner with their own private table, with their backs turned to just about everybody else ... and enjoying each other's company. And yes, we're told they were super flirty and touchy-feely.

Our sources tell us Ariana was actually the one who showed more affection here. Apparently, she kept getting up from where she was first sitting (across from ES) to come be directly by his side -- where she whispered sweet-nothings into his ear, and planted some smooches.

We're told they certainly seemed to be having a good old boozy time ... and sure, you can see they had at least a glass of wine each. In any case, our sources tell us they were there for a while ... for almost upwards of 2 hours, they were just perched there chatting.

It's interesting we're seeing them not trying to hide as much as they were the last time they were hanging out in public -- remember, they did a Disneyworld trip out in FL last month.

That was before she and Dalton were officially single in the eyes of the law -- something that wrapped up a few weeks ago ... when she and her ex settled their divorce, without much drama. In the wake of them closing that book ... DG himself seems to have moved on.