Jamie Foxx callously crushed the dreams of an up-and-coming singer … by jacking his song — “Party Ain’t a Party” — so Foxx could selfishly resurrect his own music career, according to a new lawsuit.

Foxx released the song — featuring 2 Chainz — as a single this week … “after being absent from the music scene for over four (4) years,” according to court docs obtained by TMZ.

Just one problem — it’s the EXACT same song recorded by a young artist named J Rand a year earlier! Seriously, listen … the ONLY difference between the two tracks are the singers.

Nontra Records says the rip-off is blatant and shameless — because J Rand’s version and Foxx’s version were both produced by the same guy — DJ Mustard.

Nontra’s suing all 3 guys — Foxx, 2 Chainz and DJ Mustard.