Pocono Mountain Regional Police say Bernard Pedreira shot 72-year-old Lydia Alamillo Bulaon at their home in Tunkhannock Township early Thursday.
A police affidavit says Pedreira told officers he was cleaning his guns when his girlfriend yelled at him to put them away.
Police say Pedreira told them she grabbed a loaded gun and it went off, killing her.
Court papers say he later told police he had "his finger on the trigger" at the time.
Authorities said the couple had a history of domestic complaints.
Pedreira is jailed without bail on a homicide charge.
Court records don't list an attorney who could speak on his behalf.