LADY GAGA GETTING SUED

Accused of Ripping Off 'Judas : Lady Gaga's hit song "Judas" is a total rip-off of a 1999 song called "Juda" ... so says a singer named Rebecca Francescatti ... who just filed a lawsuit against Gaga in federal court.

Rebecca claims the similarities between the two songs are NOT a coincidence ... because her former bass player -- Brian Gaynor -- is now employed by a music company that wrote 17 songs on Gaga's recent "Born This Way" album.

Rebecca's lawyer, Chris Niro, tells TMZ, "Though the songs are different styles, the composition is the same and the chorus is the same melody ... [Rebecca] is seeking recognition for what she created."

Francescatti, who sings under the name Rebecca F., is suing for unspecified damages.

We called Lady Gaga's camp for comment ... no word back so far.