The incident occurred in the 16400 block of Paramount Boulevard around 12:30 a.m.
Los Angeles County sheriff's officials said they tried to apprehend a man who was acting erratically, but when he grabbed a sergeant's gun, they were forced to shoot him.
The man was struck in the upper torso. He was transported to a hospital, where he was pronounced dead. No deputies were injured.
Investigators said the sergeant saw the shirtless man walking on the street and talking to himself. The man appeared to be on drugs, according to officials.
One witness, Ruben Estrada, said he thought the use of deadly force was excessive.
"They could've bean-bagged him, they could've Tasered him, but no, they went and shot him," said Estrada.
"When the deputies walked up to this individual, they did have other options available to them. They had a Taser out and it wasn't until this individual grabbed the deputy's gun...that the shooting occurred," said Lt. John Corina of the Los Angeles County Sheriff's Department.
Officials said the man was not armed with a weapon. The incident remains under investigation.