Loggerhead turtles, which are rarely seen in UK waters, have been spotted near Swanage.
Marine conservationists have urged any sightings be reported instead of the turtles being returned to the sea.
It is feared the animals "would struggle to survive" in cooler sea temperatures off the UK coast.
The Marine Conservation Society (MCS) had been alerted to two loggerhead turtles about 50 metres from the shore by Durlston Head.
MCS head of biodiversity and fisheries, Dr Peter Richardson, said loggerhead turtles were "rarely spotted in UK seas".
He said: "Usually, young loggerheads thrive in warmer waters, but will stop feeding when sea temperatures drop below about 15C.
"As the seas cool down these turtles will suffer from acute hypothermia.
"If they are lucky, they may strand alive on a beach, and if found in time they can be rescued and rehabilitated back to health with specialist treatment".
He added the loggerheads would most likely "perish" in the South West inshore waters, which are cooling down to below the 15C threshold.
He added: "We urge anyone who finds a turtle on UK shores to report it immediately, and try and make sure the tides don't wash them back to sea".
Residents in Dorset have been asked to report sightings of an endangered turtle, which experts say may become stranded on beaches.