The Metropolitan Police are searching for a Millwall fan who allegedly racially abused a Barnsley player at The Den on Saturday.
The alleged incident occurred after the game and came from a section of the home crowd.
Millwall said in a statement: "Millwall Football Club and its security team are working with The Metropolitan Police and Barnsley Football Club to identify a supporter accused of racially abusing a visiting player in the immediate aftermath of Saturday's game between the two sides at The Den.
"The matter will be fully investigated by the police and the club will co-operate fully.
"Any individual found guilty of racist abuse at The Den is immediately issued with a lifetime ban and also offered the opportunity to attend Millwall's 'Fan Diversity Scheme'."
Barnsley have also indicated their awareness of the alleged incident and have said they will work together with Millwall.
A Barnsley statement read: "Barnsley Football Club and Millwall Football Club are working together, along with The Metropolitan Police to identify a supporter who is accused of racial abuse.
"Millwall Football Club have fully cooperated with Barnsley Football Club on this matter and dealt with the allegation in a thorough manner.
"We have no doubt as a club, that the staff at The Den are treating this incident with the severity that is required."