A penknife or pen knife is a small folding pocket knife, originally used for cutting a quill to make a pen nib. It may have a single blade, or have a large number of blades or other tools incorporated. Over the last hundred years there has been a proliferation of multi-function knives with multifarious and often ingenious gadgets including awls, reamers, scissors, nailfiles , corkscrews, tweezers, toothpicks and so on, and the tradition continues with the incorporation of modern devices such as ballpoint pens, LED torches and USB memory drives. The most famous example of these tools is the Swiss Army knife, some versions of which number dozens of functions and are really more of a folding multi-tool incorporating a blade or two, than a penknfe with extras.

A larger folding knife, especially one where the blade locks into place, is often called a claspknife.

Anecdotes about penknives often refer to the idea that the knife includes attachments whose purpose is mysterious and improbable, for example something for "removing stones from a horse's hoof", probably referring to the spike or awl commonly found in multi-blade knives. A proper hoof pick (a very necessary tool for removing stones which could cause lameness) is, however, a blunt, strong hook which would hardly fit conveniently into a small pocket knife.