Rangers say Alfredo Morelos was racially abused by a group of supporters during the Old Firm derby win over Celtic at Parkhead.
The 23-year-old Colombian striker was sent off in the 96th minute as Rangers won 2-1 on Sunday, securing their first victory at Celtic Park since 2010.
Morelos appeared to make a gesture with his hand across his neck towards Celtic fans following his injury-time dismissal.
A Rangers spokesman said: "We believe Alfredo was racially abused and we would expect now that all measures will be taken to identify and deal with those responsible."
Celtic replied in a statement: "We are aware of the gestures made by the player towards Celtic supporters.
"We have received no reports of any racial abuse but would fully investigate any should these be brought to our attention."
Rangers moved to within two points of Celtic with a game in hand after Nikola Katic's header sealed victory.