Rihanna -- Fan Steals the Show

Rihanna gave one of her fans an "Idol" chance to sing in front of thousands ... and he nailed it.

Terah Jay was front row at RiRi's Cincinnati concert Saturday when he goaded her into waiving the mic his way. Jay delivered.

TJ's been a choir student his whole life ... and hopes the duet is his star turn that takes him to Broadway.