Romeo Santos not only ripped off a musician's song and released it as his own, but he didn't even bother to change the name ... according to a new lawsuit.
Nazim Guity claims he wrote a tune in 2011, titled "Eres Mia," and worked with a music producer and recording studio to mix and record the track ... and copyrighted the song in March 2014.
Santos allegedly collaborated with the same recording studio after they worked with Guity and he jacked the track -- so the lawsuit alleges -- and then Romeo recorded the song and even kept the name intact.
Santos -- who's now CEO of Roc Nation Latin -- released his version of "Eres Mia" in 2014 and it was a smash hit.
Guity's suing Santos, along with Sony and the recording studio.
We reached out to Romeo's team ... so far no word back.