Troops are preparing the base for the arrival of aircraft and additional soldiers later in the summer, one USA official said.
The move comes amid heightened tensions with Iran over the safety of shipping lanes in the Gulf.
"In coordination with and at the invitation of the Kingdom of Saudi Arabia, the Secretary of Defence has authorized the movement of US personnel and resources to deploy to Saudi Arabia", the US Central Command of the Department of Defence announced in a statement.
Saudi Arabia's King Salman bin Abdulaziz Al-Saud has approved hosting US armed forces in the kingdom, the Saudi Press Agency reported Friday.
BBC North America correspondent Peter Bowes says the U.S. is understood to be deploying Patriot air defence missile batteries manned by 500 soldiers to Prince Sultan Base in Saudi Arabia.
Starting with the January 1991 air war against Iraq after its invasion of Kuwait the previous summer, the United States flew a wide range of aircraft from Prince Sultan airbase, originally known as al Kharj.
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Nadia Hopley, 36, an unemployed mother of four, said she hoped that the army's presence would make the area safer for children. There have been concerns over a delay in sending troops into the Cape Flats.
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Many in Congress now question the decades-old U.S. -Saudi security alliance and oppose major new arms sales to the kingdom. Central Command said the deployments to Saudi Arabia had been approved by the Pentagon.
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The US also plans to send a squadron of F-22 stealth fighters to the base.
This movement of forces provides an additional deterrent, and ensures our ability to defend our forces and interests in the region from emergent, credible threats.
Since May, the USS Abraham Lincoln carrier group, Patriot missile batteries, a bomber task force, and thousands of troops have been deployed to the Gulf, ostensibly to counter Iranian "escalation", amid a flurry of threats to unleash fiery doom upon Tehran should anything untoward befall Americans or their allies. The US has since tightened sanctions it re-imposed on Iran's oil sector. That same day, the U.S. claimed to have shot down an Iranian drone threatening a U.S. ship, which Tehran denies. The US insisted the drone had been over worldwide waters at the time, and condemned it as an unprovoked attack.
Iran on July 19 seized a British-operated oil tanker in the strait and briefly detained another in an apparent retaliation for a seizure off Gibraltar - a British territory - of an Iranian tanker on July 4.
U.S. President Donald Trump said on Thursday that a U.S. warship destroyed an Iranian drone in the Strait of Hormuz, but Tehran has denied the U.S. assertion.
The move, agreed in conjunction with the kingdom, aims to boost regional security as tensions in the Gulf mount over Iran's standoff with the United States over sanctions and the 2015 nuclear agreement, and Tehran's seizure of two British-linked vessels in the strait of Hormuz on Friday.