Washington sees the US-built Terminal High Altitude Area Defense system as protection against North Korea's pursuit of missile and nuclear technology.
But China has warned it sees it as a threat to the balance of power in the region, fearing it would make its own nuclear deterrent less credible.
"The United States will do whatever is necessary to defend ourselves and to honor the security commitment we have made to our allies," Kerry said.
"And we will deploy as soon as possible a Terminal High Altitude Area Defense battery to our Korean ally," he added.
Washington and Seoul agreed to install THAAD this summer following repeated nuclear and missile tests by Kim Jong-Un's authoritarian Pyongyang regime.
The move angered Beijing, the main trading partner of otherwise isolated North Korea and a key player in international efforts to control the regime.
Last week, Chinese General Cai Jun told a defense forum in Beijing that THAAD is "not conducive to the peace and stability of the Korean peninsula."