Police divers have been dispatched to the River Wandle to carry out forensic tests after a "torso" was found.
Officers say the human remains belonged to Sarah Mayhew - a woman who police believe was murdered.
Last month, other body parts were found in a park in Croydon.
Detectives were called to the discovery near Applegarth Academy on April 2 and following a full analysis, Mayhew was named as the victim.
Police have named victim Sarah Mayhew after human remains were found in park in Croydon last week
Met PoliceThe 51-year-old, who is originally from Brazil, had lived on the same street for 20 years.
Locals described the "shocking" incident as "awful".
A Scotland Yard statement said: "At 14:18hrs on Tuesday, 21 May, police were called to Rawnsley Avenue, Mitcham after human remains were found in a river by contractors carrying out cleaning work.
"Officers attended and a crime scene was established.
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"Following examination of the remains, detectives informed the family of Sarah of this development.
"They continue to be supported by specially trained officers."
Two people - known to Mayhew - were arrested and subsequently charged with her murder.
Detective Inspector Martin Thorpe, who is leading the investigation, said: "My thoughts remain with Sarah’s family. We will continue to provide our support as our investigation progresses.
Last month, other body parts were found in a park in Croydon
GB News"I expect my officers to remain in the area for some time as they carry out important work at the scene.
"Two people arrested, who were known to Sarah, were later charged with her murder.
"At present we are not looking for anyone else in connection with her death. Enquiries are ongoing."