"This sale furthers US national security and foreign policy interests, and supports the long-term security of Saudi Arabia and the Gulf region in the face of Iranian and other regional threats," a statement said.
The THAAD -- which has already been supplied to Saudi Arabia's neighbours Qatar and the United Arab Emirates -- is one of the most capable defensive missile batteries in the US arsenal and comes equipped with an advanced radar system.
But the State Department said it would advise Congress that, in Saudi hands, the system would act to stabilise the situation in the Gulf and help defend US forces in the region and their allies, who face a growing Iranian missile capability.
"The proposed sale of this equipment and support will not alter the basic military balance in the region," it said.
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"This potential sale will substantially increase Saudi Arabia's capability to defend itself against the growing ballistic missile threat in the region," a statement said.
"THAAD's exo-atmospheric, hit-to-kill capability will add an upper-tier to Saudi Arabia's layered missile defence architecture."
The main US contractors who will profit from the sale are aerospace giant Lockheed Martin's space systems division and defence contractor Raytheon.