Saudi Arabia's King Salman approved hosting U.S. forces in the country to boost regional security and stability, the state news agency (SPA) reported on Friday.
The gesture comes amid rising tensions between Washington and Tehran in the Gulf that have impacted global oil markets.
On Friday, Iran said it had seized a British oil tanker in the Strait of Hormuz, but denied Washington's assertion that the U.S. Navy had downed an Iranian drone nearby earlier this week.
The decision aims "to increase joint cooperation in defense of regional security and stability and to preserve its peace" SPA said, quoting a ministry of defense official, without giving further details.
Relations between Washington and Tehran worsened last year when U.S. President Donald Trump abandoned a 2015 nuclear deal between world powers and Iran.
Under the pact, Iran agreed to restrict nuclear work, long seen by the West as a cover for developing nuclear weapons, in return for lifting sanctions. But sanctions have since been reimposed, badly hurting Iran's economy.
Trump has said he considers Saudi Arabia an important partner in the Middle East and counterweight to the influence of Iran.