A male caller reported a shooting at about 6:05 p.m. at 100 De Sabla Road, according to Hillsborough police.
Responding officers found Bradley Kleiman at the address and arrested him without incident, police said.
Police say he told them he shot his friend after the friend pulled a gun on him.
Police declined to release information about the victim but classified the incident as a homicide.
There's no word on a motive.
It's Hillsborough's first homicide since 2004.
KGO contributed to this report