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.