"A police patrol carrying out its duties in the town of Tarot in Qatif district came under fire on Friday from a nearby farm," said Eastern Province police spokesman Ziad al-Rukaiti.
Qatif police had opened "an investigation to hunt down the criminals," said Rukaiti.
The attack came one day after the same source announced that "three systematic crimes carried out by armed rioters at different times" had targetted police in the same region.
The "crimes" include two incidents in which unknown gunmen had opened fire and hurled two petrol bombs at Tarot police station, and another in which assailants opened fire on a police checkpoint.
The statement did not specify when the attacks took place while no casualties were reported in any of the incidents.
Since early 2011, mainly Shiite towns in Eastern Province have seen sporadic protests and confrontations between police and Shiites who claim they are marginalised.
Shiites are estimated to number some two million in the Sunni-dominated kingdom.
Unrest erupted after an outbreak of violence between Shiite pilgrims and religious police in the Muslim holy city of Medina in February 2011.
The protests escalated when the kingdom led a force of Gulf troops into neighbouring Bahrain the following month to help crush Shiite-led pro-democracy protests against its Sunni-minority monarchy.
Human rights groups say more than 600 people have been arrested since spring 2011, most of them in Qatif. The majority have since been released.