Police are investigating an image that appears to show a Sunderland fan directing a monkey gesture towards West Brom striker Romelu Lukaku.
The incident occurred after the on-loan Chelsea player scored in his team's 4-2 win at the Stadium of Light on Saturday.
Northumbria Police have confirmed they are looking into the gesture, with Sunderland fully co-operating.
A club spokeswoman said: "Sunderland operates a policy of zero tolerance towards racism and anti-social behaviour of an kind and naturally we will co-operate with the police fully investigating any such incidents."
Ged Grebby, chief executive of anti-racism charity Show Racism the Red Card, has welcomed the investigation.
He stated: "We welcome the news that Sunderland AFC and Northumbria Police have announced they will investigate this."