The woman was walking near Diglis Hotel at night when she spotted the crustacean moving along the riverbank.
“It scared the life out of us, running across the steps. Even the swans were scared,” she said.
Wildlife experts believe it may be a non-native Chinese mitten crab or a crab someone has brought back from the seaside and dumped in the river Severn.
The woman struggled to get a clear photo because it was nighttime and the speed at which it was moving.
A spokesperson for Worcestershire Wildlife Trust said: “It is difficult to identify the crab from the photo, but it may well be a non-native Chinese mitten crab, as native crabs only survive in saltwater.
INVADED: Could a Chinese mitten crab be lurking beneath the waters of the river Severn in Worcester? (Image: © Copyright Philip Halling and licensed for reuse under this Creative Commons Licence)
“Of course, we can’t rule out the possibility that a coastal crab has been brought back from someone’s holiday or released from a collection.
“If it is a Chinese mitten crab, this isn’t good news, as they not only have a voracious appetite for both plants and animals, but their extensive burrows cause immense damage to river banks.
“Importantly, though, they cannot breed in freshwater, so should be unable to build up a self-sustaining population this far inland.”
Mitten crabs have been spotted in numerous locations in the UK, including Cambridgeshire and along the River Thames.
CLOSE-UP: The blurry image was taken by a woman walking along the River Severn at night (Image: Supplied)
A spokesperson for the Canal and River Trust said: “If it was truly a crab seen, then, given the freshwater location, the Chinese Mitten Crab is the only species known in the UK that lives in freshwater.
“Chinese mitten crabs are an invasive non-native and have been recorded lower down in the Severn Estuary, but have not been recorded in the Severn as high as Worcester.
“If it were a case of mistaken identity and not a crab that was seen, it could have been a signal crayfish, which looks similar to a small lobster.
“Signal crayfish are another invasive non-native species, but are more widespread than the Chinese mitten crab.”