The alleged crime came to light after police received reports of a motorist driving erratically in the south-eastern town. The car was stopped at 8.20pm.
“The man in the car explained that he had killed his mother. The police than went to an address at Hovland in Larvil where they found a dead woman,” said police inspector Magnar Pedersen.
The 34-year-old suspect, identified by the police as a psychiatric patient, has been arrested and indicted over the alleged killing.
The man has previously been arrested over threats and disorderly behaviour but no action has ever been taken due to doubts about his sanity.
The dead woman’s body is to be sent for an autopsy.
Vestfold police have asked Kripos, the National Criminal Investigation Service, to aid in the technical investigation.
Police were unable to reveal the time of death or give details about a possible motive.