Nine people were wounded in the attack in al-Dalwah town in al-Ahsa district, a police spokesman was quoted as saying.
"As a group of citizens was leaving a building...three masked men opened fire at them with machine guns and pistols," the spokesman said, adding that the incident was under investigation.
It gave no further details.
Videos purporting to show the aftermath of the attack posted to social media showed a body lying in a pool of blood outside a building, with people milling around calling for help. The authenticity of the videos could not immediately be confirmed.