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.

I think this is a bunch of bull, and this Rebecca person is just looking for some money. Judas was released months ago, and we are just now hearing about this? I do know that if Lady Gaga had known she would have just paid the woman, so I see no reason to take her to court. Rebecca is from Chicago and wrote this song back in 1999, and it's not on YouTube,  so Lady Gaga wins! lol