Cantonal police said they were contacted at around 3.40pm about an accident in the Solduno neighbourhood of the town.
The woman was reportedly “tinkering” around the opening for the sewer when for reasons not immediately known she fell into the sewer, police said in a news release.
The Locarno newspaper Giornale del Popolo reported that the woman had lost her balance.
A neighbour called emergency services after seeing the woman’s body protruding from the manhole, the newspaper said.
The woman was pronounced dead when the Locarno ambulance service (SALVA) arrived on the scene, according to police.
Forensic investigators who came to the site of the accident are trying to reconstruct what happened.