Sunderland have promised "the strongest possible action" after Romaine Mundle said he had been targeted by online racist abuse.
The 21-year-old midfielder shared a discriminatory post sent to him on his Instagram account after Saturday's 1-0 home defeat by Championship strugglers Hull.
Sunderland said they had a zero-tolerance policy against all forms of discrimination and vowed to track down and punish those responsible.
"The club will continue to work with the authorities and the EFL to ensure the well-being of our players and staff," they said in a statement.
"We will also continue to report all forms of online discrimination to [Instagram owners] Meta and urge them to do more to remove this unacceptable behaviour from their platforms.