Ratnakar Ugalmugle, posted with district police headquarters here, was known to the victim, a widow, and often visited her home, police said today.
Ugalmugle had promised to marry her, but did not keep his word, they said.
According to a complaint lodged by the victim last night, he sexually assaulted her several times during the last about 20-25 days.
An offence of rape has been registered against the Constable, Tarapur Police Station in-charge Durgesh Shelar said.
Ugalmugle is yet to be arrested.