Cincinnati Reds announcer Thom Brennaman uttered an offensive, homophobic slur while calling the Reds game Wednesday ... apparently not realizing his microphone was on.
Brennaman -- who was announcing a doubleheader against the Kansas City Royals -- was heard referring to a city as "one of the f** capitals of the world."
Seconds later he snapped back into announcer mode and read a promo for the Reds Live Pregame Show ... as if nothing had happened.
But viewers were SHOCKED ... and the clip went viral on social media in a matter of minutes.