Koulibaly has made 149 appearance for Napoli since joining in 2014
Arsenal have launched an investigation after a social media video appeared to show a fan racially abusing Napoli's Senegalese defender Kalidou Koulibaly.
The supporter can be heard using a racial slur in the video, filmed during Arsenal's 2-0 Europa League quarter-final first-leg win on Thursday.
"We utterly condemn use of this type of racist language," Arsenal said in a statement on Friday.
The club reiterated they have a "zero-tolerance" approach to such behaviour.
"We have launched an investigation to identify the culprit," the statement added.
"We operate a zero-tolerance approach and anyone behaving like this is not welcome at Arsenal and will be banned from matches.
"We have an extremely diverse community of fans who are all part of the Arsenal family and such incidents are rare at the Emirates Stadium.
"We encourage supporters to report any incidents as they happen to stewards or via our matchday alert service."
On Thursday, Chelsea barred three fans from entering Slavia Prague's stadium after video footage showed them calling Liverpool's Mohamed Salah a "bomber".
