The shark can be seen swimming near boats with its vibrant blue body glistening in the water.
Local artist Rob Young captured the incredible images while out walking along the harbour, much to his surprise.
He said: “I was surprised to see the shark.
“I’d never seen one this close to shore before.
“Maybe it was after the pollock under the boats or just momentarily disoriented and acting out of its normal behaviour patterns.”
You can watch the video below.
The blue shark is an open-ocean species that visits UK seas in summer months.
In the Atlantic, they appear to follow a clockwise route, following the Gulf Stream to the UK from the Caribbean and returning there following the Atlantic North Equatorial Current.
They are active predators and feed mainly on small fish and squid – though they have been known to take seabirds and other small sharks too. They even feed on fish that live near the seabed and have been recorded at depths of up to 350m.
So, seeing the shark in the harbour is a rare sight.
The Dorset Wildlife Trust confirmed the species after experts reviewed the video footage and provided more detail about the animal’s habits.
Marine engagement assistant for the trust, Hayley Roberts, said: “From the footage, we can confirm this is a blue shark.
“This species has an elongated, slender shape, a long conical snout, long pointed pectoral fins, and a metallic blue body.
A blue shark spotted swimming in Weymouth Harbour (Image: Rob Young)
“Blue sharks, particularly a dominance of females, are usually present in UK waters from May to October, when water conditions are favourable and there is an abundance of prey.
“They are usually spotted well offshore, so to see one in a harbour is quite rare, although it does happen from time to time.
“In recent years there has been an increase in blue shark sightings, alongside other shark species such as porbeagle and thresher sharks, off the south coast.
“There are several possible reasons for this, including increased protection and changes in ocean conditions, such as warming seas, although the exact causes are still being investigated.”
It is the second shark sighting this month after beachgoers captured pictures of it swimming close to shore in the shallows at Weymouth beach.
The Shark Trust said this was likely a basking shark, although a conclusive identification was not possible due to the limited information available from images.