The shark was seen in shallow waters off Smeaton's Pier in St Ives on Saturday afternoon by Tristan Campion. His wife Holly told Cornwall Live that children started shouting "It's a shark! It's a shark!" as they were jumping off the pier into the water.
Tristan managed to get video footage of the shark in the water, and uploaded it to Facebook.
However, sun seekers needn't have worried - while the shark, thought to be a porbeagle, is related to the great white shark, it is actually not a danger to humans.
Some people suggested the animals was a young basking shark.
According to the Metro, Colin Walker, maritime assistant at St Ives Harbour, confirmed it was a porbeagle, and said: "We hardly ever see them here, let alone so close in but there were also a number of dolphins in the water on the same day.
"It was an amazing sight."
The porbeagle typically reaches 2.5 metres long (8.2ft).
It is an opportunistic hunter that preys mainly on bony fishes and cephalopods throughout the water column, including the bottom. Most commonly found over food-rich banks on the outer continental shelf, it makes occasional forays both close to shore and into the open ocean to a depth of 1,360 m (4,460 ft). It also conducts long-distance seasonal migrations, generally shifting between shallower and deeper water.
The porbeagle is fast and highly active, with physiological adaptations that enable it to maintain a higher body temperature than the surrounding water.
It can be solitary or gregarious, and has been known to perform seemingly playful behaviour. Females typically bear four pups every year.
Only a few shark attacks of uncertain provenance have been attributed to the porbeagle.