Garden City police informed that the man, whose name hasn't been revealed pending notification of family, died at a local hospital soon after he was taken there.
Officers responded to a drowning report at about 5:05 p.m. in the area of East 36th Street and the Boise River near Whitewater Park.
Witnesses said they called 911 after they saw a man in distress floating downstream.
After search began, someone on the Greenbelt informed officers they saw what appeared to be a body floating near East 40th Street.
Emergency personnel were unable to access the area from the riverbank due to the water level, so the Boise Fire Department provided its rescue boat and dive team and searched the area.
The man was finally located near East 41st Street at around 6 p.m. and was pulled from the river. Emergency personnel began life-saving measures.
He was then taken by ambulance to a hospital.
Police continues to investigate the incident.