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.
"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," a statement from US Central Command said.
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.
BDST: 1712 HRS, JUL 20, 2019