Warm waters off the Sussex coast brought a dolphin to the gates of Eastbourne's harbour.
Concerned members of the public and staff at Sovereign Harbour called wildlife rescue groups who went to the scene to ensure the animal was not injured.
The British Divers Marine Life Rescue director Alan Knight was joined by Trevor Weeks of the East Sussex Wildlife Rescue and Ambulance Service (WRAS) on Monday night.
The dolphin appeared to be circling in front of the lock but after observing the mammal for several hours it was thought to be fit.
Mr Weeks said: "We were concerned the geography of the site might be a problem and cause the dolphin to become stranded."
He left the harbour at 2.15am yesterday and returned at 6.50am to check the animal had left safely.
Marine Mammal Medics from British Divers Marine Life Rescue and WRAS will keep an eye on the area.
Anyone who spots a dolphin and thinks it could be in distress should contact British Divers Marine Life Rescue on 01825 765546.
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