One of Tupac's hits from the 90's features a singer who says she never got paid for her vocals, so she's suing ... even though it appears 20 years too late to collect.

Natasha Walker -- who was in the R&B group Y?N-Vee -- sang backups on "Bury Me a G" and even has a credit on the track. But in docs, obtained by TMZ, she says she and Pac made a deal in '93 that the song would be a joint effort and she'd have equal ownership.

Natasha says she's never seen a dime from the song's release ... hence her lawsuit. One potential problem for her: It's 2017 and the statute of limitations for contract claims in California is just 4 years.

Walker's attorney, Jennifer Harris, tells us because the song continues to make money and will for the foreseeable future ... her client has every right to come-a-knockin' now. She wants all royalties, back and current, plus damages and attorney's fees.

We've reached out to Tupac's estate ... so far, no word back.