Police Scotland responded to a report of a man entering the River Nairn, Harbour area, at 10.56pm on Saturday.
Officers found two men in the water with one attempting to help the other.
One man managed to get out of the water but the other was lost from sight.
The missing man is 19 years old and local to the area, police said.
Coastguard, Invergordon and Moray Lifeboat, Scottish Fire and Rescue Service, Scottish Ambulance Service and an RAF rescue helicopter attended the scene and carried out a search of the area.
The search resumed at 8am with police deploying specialist resources, including the Underwater Diving Unit.
A body was discovered by police divers this afternoon.
Formal identification will take place in due course and a report will be submitted to the Procurator Fiscal.