The fighter jets contract is the biggest deal so far signed under the military's long-delayed modernization program.
The signings come amid tensions with China in the South China Sea.
"With the eventual delivery and acquisition of these new air assets, our air force can already forget the lingering naughty joke that it is all air without force," said Defense Secretary Voltaire Gazmin, who witnessed the signings.
The FA-50s serve as jet trainers, lead-in fighters and multipurpose fighters, according to Korean Aerospace Industries' president and chief executive officer, Ha Sung Yong.
Air Force spokesman Col. Miguel Ernesto Okol said the acquisition of the fighter planes "would signal the start of our territorial defense initiatives" and "brings us closer to what we plan to achieve, which is credible defense capability."
The air force has had no fighter aircraft in its inventory since 2005, when a fleet of F-5 jets was decommissioned.