The interceptors fired from THAAD's launcher do not carry warheads and instead use pure kinetic energy to deliver "hit to kill" strikes to ballistic threats.
North Korean leader Kim Jong Un spoke out against the bilateral decision by Washington and Seoul to deploy THAAD on the Korean peninsula.
After the announcement, the North test-fired a KN-11 submarine-launched ballistic missile in retaliation.
Under third-generation North Korean leader Kim, the reclusive state has conducted its most powerful nuclear test, launched its first-ever intercontinental ballistic missile and threatened to send missiles into the waters near Guam.
Since 2011, Kim has fired more than 85 missiles and conducted four nuclear weapons tests, more than his father, Kim Jong Il, and grandfather, Kim Il Sung, launched over a period of 27 years.
The Pentagon contract announcement comes on the heels of U.S. President Donald Trump's confirmation Wednesday that CIA Director Mike Pompeo met with Kim last month to lay the groundwork for a potential summit between Kim and Trump.
The widely anticipated meeting would be the first between sitting leaders of North Korea and the United States.