Ariana Grande and Ethan Slater Living Together As Relationship Gets Serious

Ariana Grande and Ethan Slater are in an increasingly serious relationship ... because TMZ has learned they are now living together.

Sources with direct knowledge tell TMZ ... Ariana and Ethan are living under the same roof in New York City. That said, we're told each maintains a separate residence, but they are together almost all of the time.

One source tells TMZ, aside from sharing a common interest in acting ... they "vibe together" and enjoy each other's company to the point they'd rather spend time together as a couple than galivante around town to fancy restaurants.

We're told Ariana will return to England early next year to finish filming "Wicked." It's unclear whether Ethan will go ... he has a commitment on Broadway where he stars in "Spamalot." Even if they are on separate continents while she finishes the film, we're told she'll only be in England for a short period of time.

We're told the plan will be status quo when Ariana returns to NYC, underscoring this relationship is serious.