In a clip that has circulated around social media, Brennaman was heard saying the slur moments after the Fox Sports Ohio broadcast returned from a commercial break before the top of the seventh inning. The 56-year-old sportscaster didn't appear to realize he was already on the air.
"One of the [expletive] capitals of the world," Brennaman said on air.
In the fifth inning of the second game of a doubleheader, Brennaman issued an apology for using the slur. He said he was "deeply ashamed" before signing off and turning over play-by-play duties to Jim Day.
"I made a comment earlier tonight that I guess went out over the air that I am deeply ashamed of," said Brennaman, who interrupted his apology after a Nick Castellanos home run. "If I have hurt anyone out there, I can't tell you how much I say from the bottom of my heart I'm so very, very sorry. I pride myself and think of myself as a man of faith.
"I don't know if I'm going to be putting on this headset again. I don't know if it's going to be for the Reds. I don't know if it's going to be for my bosses at Fox. I will apologize for the people who sign my paycheck, for the Reds, for Fox Sports Ohio, for the people I work with, for anybody that I've offended here tonight."
Thom Brennaman -- the son of Hall of Fame broadcaster Marty Brennaman -- has called MLB games for 33 years and has been a member of Fox Sports for the last 27. He has primarily covered baseball and football.