Four Saudi officers were shot dead and four others wounded on Thursday when their checkpoint came under gunfire in southern Asir province, state media said.
Three officers were instantly killed when the outpost came under attack, the Saudi Press Agency reported, without specifying who was behind it.
"Security officials have succeeded in identifying a number of suspects involved in the crime and arrested two of them, both Saudi nationals," SPA said.
"In the interest of the investigation, their names cannot be revealed."
A third Saudi suspect opened fire as he tried to escape, killing an officer and wounding four others, SPA added.
Security forces fatally shot the suspect, who was identified by Saudi media as Bandar Mohammed Ali al-Shahri.
The attack comes as Saudi Arabia is embroiled in a long-running conflict in its southern neighbour Yemen, dubbed by the United Nations as the world's worst humanitarian crisis. Asir province borders Yemen.
In March 2015, the kingdom assembled a coalition of Arab states to fight the Houthi rebels in Yemen. The Houthis did not claim responsibility for Thursday's attack.
Since late 2014, the Islamic State (IS) group has claimed a series of bombings and shootings against security forces and Shia houses of worship in the kingdom.