The short-snouted variety are endangered and normally live around the Canary Islands and Italy.
Experts at London Zoo said the species had been found at Dagenham in east London and Tilbury and Southend in Essex, over the last 18 months.
The revelation coincided with new laws which came into force on Sunday to give the creatures protected status.
The seahorses, or Hippocampus hippocampus, are now protected under the Wildlife and Countryside Act 1981.
They are usually found in shallow muddy waters, estuaries or seagrass beds and conservationists said their presence in the Thames is another good sign that the water quality of the river was improving. – bbc