Saudi Arabia confirmed that King Salman had approved the move "to strengthen regional security and stability".
An official in the Saudi Ministry of Defence clarified that the decision was made based on the principles of strengthening regional security and stability, and joint cooperation between the two countries.
Since Washington withdrew from the 2015 nuclear agreement, tensions have risen between the US and Iran.
Iranian Foreign Minister Mohammad Javad Zarif called the USA plans a "symbolic move" that will not bring benefits to any country in the region in terms of security.
The U.S. army left Saudi Arabia in 2003 after a twelve-year presence that started when Iraq invaded Kuwait in 1991.
The Pentagon said on Friday that the move would provide "an additional deterrent" in the face of "emergent, credible threats".
In response to attacks on four other tankers on May 12, United States officials announced that month plans said to send 900 more forces - including engineers and a fighter aircraft squadron - to the Middle East to bolster U.S. defence, as well as extend the deployment of some 600 personnel manning Patriot missiles.
The announcement of the US force movement came at a moment when the regional situation is volatile.
Saudi Arabia has rolled out the welcome mat for U.S. troops in a move it claims is aimed at "preserving its peace" - mainly by menacing Iran, where tensions have risen near a breaking point after multiple tanker seizures.
Last month, Iran shot down a U.S. surveillance drone over the Strait of Hormuz, accusing it of violating Iranian airspace. But senior USA defense officials told AP that some troops and Patriot air-defense missile systems had already arrived at Prince Sultan Air Base, south of Riyadh and site of the previous US deployment.
The British-flagged, Swedish-owned Stena Impero was carrying 23 crew members at the time of its capture, according to the company.
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.
Relations between Washington and Tehran worsened previous year when President Donald Trump abandoned a 2015 nuclear deal between world powers and Iran. While Trump claimed the United States shot down - or electronically jammed - an Iranian drone last week, Iran released a video on Friday appearing to show no drone plummeting into the ocean during the relevant time window.