The Pentagon has said US troops are being deployed to Saudi Arabia to defend American interests from "emergent credible threats".
The move comes amid heightened tensions with Iran over the safety of shipping lanes in the Gulf.Saudi Arabia confirmed that King Salman had approved the move "to strengthen regional security and stability".
The kingdom has not hosted US forces since 2003 when they withdrew at the end of the war with Iraq.
The US presence in Saudi Arabia started with Operation Desert Storm in 1991, when Iraq invaded Kuwait.
BBC North America correspondent Peter Bowes says the US is understood to be deploying Patriot air defence missile batteries manned by 500 soldiers to Prince Sultan Base in Saudi Arabia.
The US also plans to send a squadron of F-22 stealth fighters to the base.
What's the background?Tensions between the US and Iran have worsened since Washington unilaterally withdrew from a landmark 2015 nuclear deal. The US has since tightened sanctions it re-imposed on Iran's oil sector.
Last month, Iran shot down a US surveillance drone over the Strait of Hormuz, accusing it of violating Iranian airspace. The US insisted the drone had been over international waters at the time, and condemned it as an unprovoked attack