The direct commercial sale calls for the systems to be installed on the 2,300-ton frigate-class ships starting in April 2013.
"Phalanx is uniquely suited for the littoral environment in which the Korean fleet operates," said Rick Nelson, vice president of Raytheon Missile Systems' Naval Weapon Systems product line.
"Thanks to its multi-spectral sensor suite, Phalanx can terminally engage all threats in a multi-path, land clutter and glint environment. With Phalanx on board, the South Korean fleet will have the best possible ship self-defense available."
Phalanx is a rapid-fire, computer-controlled radar and 20mm gun system that automatically acquires, tracks and destroys enemy threats that have penetrated other ship defense systems. More than 890 systems have been built and deployed in the navies of 25 nations.
South Korea's FFX ship class will replace the Ulsan and other smaller frigates. Ulsan-1 is designed for anti-ship, anti-submarine and anti-aircraft warfare.
Raytheon issued the announcement in Britain, where it is showcasing products at the Defense and Security Equipment International trade show.