Security forces opened fire with live rounds after protesters hurled stones at one of their vehicles in the Shiite village of Al-Awamiya in the Qatif region, witnesses said.
Activists said Issam Mohammed, 22, was killed by multiple bullet wounds.
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Three other people were wounded by security force fire, one of them a man driving through a checkpoint at the entrance to the village, the activists said.
Security forces sealed off the village after the clashes, witnesses said.
Protests erupted in the Eastern Province in March when members of the kingdom's Shiite minority took to the streets to condemn Saudi military intervention against Shiite-led pro-democracy demonstrations in neighbouring Bahrain.
Four Shiites were shot dead in November. The interior ministry said security forces had come under fire from gunmen operating on "foreign orders," in a veiled accusation against Shiite Iran.
A total of 400 people were arrested, of whom around 70 remain in custody, according to activists.
Most of Saudi Arabia's estimated two million Shiites live in the Eastern Province. They complain of marginalisation in the Sunni-dominated kingdom.
