"We are concerned, the threat level has gone up and in light of the threat level and the information that we have received ... we will be having an ordered departure," Powell said on Thursday.
The order will also require the family members of all employees at the US embassy in Riyadh as well as the consulates in Dhahran and Jeddah to leave Saudi Arabia, State Department officials said.
The decision marks the third time in the last 12 months that US diplomats had been ordered to leave Saudi Arabia because of potential threats.
On Tuesday, after a clash between Saudi authorities and suspected armed opponents in the capital in which six people were killed and two explosives-laden cars discovered, the US embassy in Riyadh warned Americans in Saudi Arabia of the possibility of new attacks.
"The US government continues to receive credible indications of terrorist threats aimed at American and Western interests in Saudi Arabia, including the targeting of diplomatic and other official facilities and residential compounds in Riyadh," the embassy told US citizens in the kingdom.
A day later, a member of the Saudi security forces was killed and another wounded by gunmen who fired on their patrol northwest of Riyadh, the latest in the series of incidents.
Saudi authorities blamed the attacks on "members of a deviant minority", which usually refers to suspected Islamist sympathisers of Saudi-born Usama bin Ladin's al-Qaeda network.
Many suspected anti-government activists, as well as security personnel, have been killed in gun battles, particularly in Riyadh, since a series of bombings targeting residential compounds in the capital killed 52 people in May and November 2003.