TMZ obtained video of Ethan glued to Ariana's side as they were deep in convo during a swanky dinner at Nobu Caesars Palace. They took turns putting their arms around each other during the meal ... which was hosted by Universal after the couple -- and "Wicked" costars -- arrived at CinemaCon to hype the movie.

While it was, technically, a work event, they were still all about smiles and sweet nothings -- she even caressed his back while they chatted, with their eyes locked on each other.

Mind you, the event was packed with other "Wicked" stars, including Michelle Yeoh, Jeff Goldblum and Cynthia Erivo ... but Ari and Ethan were laser-focused on each other.

We're told they did try to keep a low profile, at least while entering -- their security team whisked them in through a back door, before seating them at a table near the entrance.

As we reported ... Ariana and Ethan's shared time in Oz, on the "Wicked" set, sparked a real-life romance starting in July 2023.

It all came on the heels of Ariana's split from Dalton Gomez and Ethan ending his 4-year marriage to Lilly Jay -- which turned into a whirlwind of scrutiny.

But, Ariana turned the criticism into inspo for her new track "Yes, And?" earlier this year ... which basically plays like a masterclass in tuning out haters.