The Chicago Cubs launched an investigation on Wednesday after a fan was caught making a racist gesture behind an African-American television reporter during a broadcast.
In a strongly-worded statement, the Cubs said any fan found guilty of racist behaviour would be banned for life from Wrigley Field.
The statement came after the incident on Tuesday, when a fan standing behind NBC reporter Doug Glanville was seen making an upside down "OK" sign, a gesture which is associated with white supremacist movements.
"An individual seated behind Mr Glanville used what appears to be an offensive hand gesture that is associated with racism," Cubs president of business operations Crane Kenney said in a statement.
"Such ignorant and repulsive behavior is not tolerated at Wrigley Field. We are reviewing the incident thoroughly because no one should be subjected to this type of offensive behavior.
"Any derogatory conduct should be reported immediately to our ballpark staff. Any individual behaving in this manner will not only be removed from the ballpark, but will be permanently banned from Wrigley Field."
Kenney told WSCR-AM 670 sports radio in Chicago on Wednesday that the Cubs were certain the gesture had been made with racist intent, dismissing suggestions that the fan may have been playing a version of the 'circle game', a playground game where participants attempt to get others to look at a hand gesture inadvertently.
"We reached the conclusion that it's more likely than not that this person was using that hand signal as a racist way of interfering with everyone's enjoyment of the game," Kenney said.
"Whether this person is going to ultimately say he intended it, that he was playing 'the circle game' or some other stunt, the judgment to use that in connection with a respected reporter who happens to be African-American doing his job ... that coincidence is not going to fly here," Kenney added.