Newcastle and Sammy Ameobi have called in police after the striker was the subject of racist abuse on Twitter.
The comment was made on Sunday and the offending account has now been deleted.
A statement said: "The club and Sammy have reported the matter to the police, who are now dealing with the incident.
"The racist comment is wholly unacceptable. Newcastle will not tolerate racism of any kind and will take the strongest possible action against those responsible."
Ameobi, 19, the younger brother of Shola, appeared as a late substitute in Monday night's 3-1 Premier League victory at Stoke.
Newcastle's statement comes at a time when the Football Association are investigating allegations that QPR defender Anton Ferdinand was the subject of a racist slur by Chelsea captain John Terry.