The Bangkok Post reported that Boonsong Kowawisarat, 55, and Chanakarn Detkard, 46, who is also his secretary, were enroute to Bangkok on Sunday when they stopped to eat in the northern province of Phrae.
While they were waiting for their food, Kowawisarat took out a 9mm Uzi pistol that discharged, hitting Detkard in her stomach.
Police said the owner of the restaurant took Detkard to hospital, where she was pronounced dead. Kowawisarat was said to be in shock after the shooting.
The senator said the shooting was an accident and that he had been trying to clear a stuck bullet from the weapon when it fired, AFP reported.
"It was my gun. I showed her the gun that I had bought for her. She lived with her mother and our two daughters - there are only women in her place - so I wanted her to have it as I'm worried about them," he told AFP.
"My hand is not in good condition. I am ill. I had a stroke," he said.
While police say they will charge the senator with causing death by negligence, they have yet to arrest and charge him as he is protected by parliamentary privilege while the country's house is in session.
Kowawisarat could face a maximum of 10 years behind bars and a fine of about 20,000 Thai baht ($632 Cdn) if he is convicted.