A person who was pulled unconscious from the river near Trelissick gardens is understood to have died on Wednesday evening.
Falmouth coastguard officers and paramedics were called to Cowlands Creek just after 4pm, where they found a person had been taken from the water.
A coastguard spokesperson said: "Basic life support was administered by coastguard officers and the casualty was stretchered to a nearby ambulance for further treatment."
An air ambulance was called, but the casualty was transported to the Royal Cornwall Hospital by road.
The spokesperson added: "Our thoughts are with the family and friends of the casualty at this sad time."
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