The Common dolphin was discovered on the high water line at Gyllingvase near Falmouth early on Wednesday.
Members of British Divers Marine Life Rescue covered the animal with wet towels and seaweed to keep it comfortable.
A vet who was called to the scene assessed that it was too sick to be returned to the sea.
There have been 41 strandings of Common dolphins in Cornwall in the past year according to figures from the Marine Strandings Network.
Almost 3,500 whales, dolphins and porpoises have been stranded on UK coastlines over the six years up to December 2011, according to the Zoological Society of London (ZSL).
Overall it recorded a fall in the number of strandings but said that some still were not being reported.
The Society has launched a campaign to encourage the public to tell them when they find beached mammals.
A dolphin found stranded on a Cornish beach has been put down, say marine rescuers.