A woman was assaulted from behind in Edithna Street, South Lambeth, at about 17:40 GMT on Saturday.
The suspect fitted the same description given to the Met Police by victims in the previous eight incidents.
Two men have been arrested so far in the investigation and police have increased their patrols in south London in response to the attacks.
In each case the victims were white women in their 20s and 30s and the suspect was described as having a Mediterranean or Middle-Eastern appearance with dark hair.
In the latest case, police said the man "touched the woman's backside multiple times".
Chief Insp Tony Hirst urged all women in the Clapham and Brixton Hill areas to "take sensible precautions".
"If you feel uneasy about an individual or situation, trust your instincts. Always call the police if you feel in danger," he said.
Two men aged 34 and 30 have been arrested and bailed in connection with the investigation.
Police investigating a string of sexual assaults in south London say a ninth woman has been attacked.