A butterfly knife, also known as a balisong, fan knife or Batangas knife, is a type of folding pocketknife that originated in the Philippines. Its distinct features are two handles counter-rotating around the tang such that, when closed, the blade is concealed within grooves in the handles. A latch sometimes holds the handles together, typically mounted on the one facing the cutting edge (the "bite handle").

The balisong was commonly used by Filipinos, especially those in the Tagalog region, as a self-defense and pocket utility knife. Hollow-grind balisongs were also used as straight razors before conventional razors were available in the Philippines. In the hands of a trained user, the knife blade can be brought to bear quickly using one hand. Manipulations, called "flipping", are performed for art or amusement. Blunt "trainer" versions of these knives are available and can be used to practice tricks without the risk of injury.

The knife is now illegal or restricted in some countries, often under the same laws and for the same reasons that switchblades or concealed weapons are restricted. Within the Philippines, it is no longer as common in urban areas as in the past.