The short-snouted seahorse found during a routine fish survey at Greenwich was a juvenile, which experts say could mean a colony of the fish could be living nearby.
The Thames had been so contaminated by the industrial revolution that it was considered "biologically dead” as late as the 1950s.
But environmental and conservation efforts have turned it into a far cleaner waterway today, one that is hosting a widening variety of marine life.
The 2-inch seahorse discovered in Greenwich was measured and quickly released back into the river.
The last sighting of a seahorse in the Thames was much farther downstream in Dagenham in 2008.
Photo: (UK) Environment Agency