Rihanna better run for cover ... because she just made Charlie Sheen's s*** list in the worst way possible -- after rejecting Charlie's request to introduce his fiancee to the singer.
Charlie just launched into a tirade on Twitter, describing his sour encounter with Rihanna Wednesday night at the ritzy Giorgio Baldi restaurant in Santa Monica.
Charlie said he was there celebrating the birthday of his fiancee Scottine (formerly Brett Rossi) ... and when he found out Rihanna was there too, he sent a request to the singer's table for a meet-and-greet ... because Rossi's a big fan.
But Charlie says his request was DENIED -- "there were too many paps outside" -- and now, the Warlock is back with a vengeance.
Sheen wrote, "Nice impression you left behind, Bday or not. Sorry we're not KOOL enough to warrant a blessing from the Princess (or in this case the Village idiot)."
Then he unloaded on Rihanna's wigged out get-up -- "Halloween isn't for a while. but good on you for testing out your costume in public. it's close; a more muted pink might be the answer, as in: none."