South Korea is to pay 7.3 trillion won ($7 billion) for 40 F-35A fighter jets under the terms of a deal with Lockheed Martin announced Wednesday by the state arms procurer.
Seoul has been in talks with the US defence firm on terms of the deal since March after Seoul picked the F-35A as its next-generation fighter jet.
"We held negotiations from March to September on technology, price and trade-off conditions", a spokesman of the Defense Acquisition Program Administration said.
"The purchase price per unit is around 120 billion won," he was quoted as saying by Yonhap news agency.
South Korea wants to start deploying the new combat planes from 2018, he said.
As part of the deal, Lockheed Martin will transfer technologies in 17 sectors including flight control and fire-extinguishing functions, Yonhap said, quoting unnamed DAPA sources.
The South's air force has been looking to retire its ageing fleet of Boeing F-4 and F-5 fighters and replace them with new jets.
The absence of a stealth capability was behind the military's decision in September last year to block a $7.7 billion deal to buy 60 of Boeing's F-15 fighters.
The Boeing aircraft was the only one to come in under the $7.7 billion budget approved by parliament.
The tender has since been adjusted.
The F-35A takes off and lands conventionally. The F-35B variant can land vertically, like the obsolete Harrier, and its users will include the US Marines and Britain's armed forces.