According to legal docs, obtained by TMZ, Ethan Slater decided to legally end things Wednesday with Lilly Jay ... filing divorce papers in New York. We don't yet know the specific reason Ethan's listed for the split.

Ethan and Lilly were high school sweethearts -- getting hitched back in 2018 -- and welcomed a son together last year.

The move isn't necessarily surprising, TMZ broke the story ... Ariana and Ethan have been together for several months now, sources told us both were single when they started dating after meeting on the set of "Wicked."

However, sources close to Lilly told us she was heartbroken to hear about Ethan's new relationship ... and feels he abandoned his brand-new family.

As for Ethan and Ariana's side of Lilly's claims, a source close to the two told us, "It's understandable that emotions are high and it's hard seeing your ex move on, especially in such a public way, and her friends are trying to protect her. But Ari and Ethan are just trying to lay low and be respectful of their exes as they pursue this new relationship."