The US government has approved the sale to Saudi Arabia of its advanced Terminal High-Altitude Area Defense (Thaad) missile defence system.
The State Department said the $15bn (£11.5bn) deal furthered US national security and foreign policy interests.
It would boost Saudi and Gulf security against Iranian and other regional threats, the state department added.
The announcement comes a day after Saudi Arabia agreed to buy air defence systems from Russia.
The deal would not alter the military balance in the region, the Pentagon’s Defense Security Co-operation Agency said.
Thaad systems are being deployed in South Korea to protect against a possible missile attack from North Korea.
But many South Koreans have objected, fearing it would become a target and endanger the lives of those who live near its launch sites.
China also voiced opposition to the system, saying it would affect the regional security balance.
The system destroys incoming missiles at altitudes beyond the Earth’s atmosphere, making it especially useful in countering missiles that might carry a nuclear warhead.
The Thaad interceptor is produced by the US company Lockheed Martin. (Courtesy BBC)