Killzone: Mercenary is a first-person shooter video game and the second handheld game in the Killzone series. For the first time in a Killzone campaign, players can fight alongside Helghast forces as well as ISA specialists, carrying out missions that regular soldiers cannot. As a mercenary, players are free to decide which tactics and load outs they will use to fulfill their contract; employers will reward players with money if successfully completed. The game utilizes the PS Vita's touchscreen and rear touch panel.

Starting just after the ending of the original Killzone, and revisiting many of the events from the Killzone trilogy, the game puts players in the rugged combat boots of a mercenary named Arran Danner. Danner, a former UCA soldier, has no qualms about taking on paid contracts from the ISA, as well as the Helghast.