Killing Floor is a first person shooter, allowing each player to move through a 3D environment. Gameplay consists of two game types, in which the player fights waves of zombie-like specimens, with each wave becoming successively more difficult, until it concludes with a battle against a "boss" specimen, called the Patriarch. Players earn in-game money for each kill, as well as for surviving to the end of a wave. In the time between each waves, players can visit a trader to buy and sell ammo, armor, and additional weapons, such as katanas, pipe bombs, flamethrowers, shotguns, and so forth; the Trader's location on the level varies at the end of each wave, discouraging players from camping in one location. Weapons may also be found randomly across the level. Players are encouraged to work together; they can trade items and drop money, healing is more effective on other players than on oneself, and the team can strategically weld doors shut to provide a temporary barrier from the oncoming horde while funneling the other creatures to specific areas.
Players select one of seven perks at the start of a match and between rounds. The perk, similar to the idea of a character class, provides bonuses towards certain weapon types, armor, movement, and other factors. Players can level up each perk to increase the benefits it offers. For example, the "Sharpshooter" perk, which provides accuracy benefits when using scoped rifles, can be raised in level by completing a number of headshots with specific weapons. The player does not need to play with that perk to raise its level; for example, the Field Medic perk requires the player to heal a number of points to level up, but this can also be done while playing any of the other perks.