A balisong, also known as a butterfly knife, fan knife or Batangas knife, is a type of folding pocketknife that originated from 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"). An exceptionally large balisong is called a balisword.

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

The knife is either illegal, or the length of the blade is restricted in some countries. Often restricted under the same laws and reasons as switchblades or concealed weapons, additionally having restrictions in some countries due to the balisong being classified as a gravity knife. Within the Philippines, it is no longer as common in urban areas as in the past.

Names for the knives in English include "fan knives" and "butterfly knives" from the motion, and "click clacks" from the sound they make when they are opened and closed.

The name "balisong" is derived from barangay (village) Balisong, part of the municipality of Taal, Batangas province, which along with the neighboring barangay Pandayan (now part of Poblacion zones 6, 9, and 10 of Taal town proper), were the original manufacturing centers of the knives in the Philippines. The two barangays were home to a blacksmith industry that also produced other bladed implements such as bolo knives. It is also dubiously claimed that the meaning of the term balisong is derived from the Tagalog words baling sungay (literally, "broken horn") as the hilt of the blade was supposedly traditionally made from carved carabao and deer horn, as well as bones, but the mass production of the knife ever since the early 20th century already used metal. The traditional balisong is also known as veinte y nueve or "twenty-nine" in the Philippines because they are 29 cm (11 in) long when opened.

The origin of the knives is unclear. Mass production of the balisong in the Philippines can only be attested to the early 1900s. It is speculated that the balisong was originally an adaptation of a French measuring tool called the pied du roi ("foot of the king"), invented between the 1500s to the late 1760s. However, how it was introduced to the Philippines is unknown. There are theories that it may have been introduced by sailors in the Spanish Empire, which was then allied with France.

Regardless of origin, the modern balisong was perfected in the Philippines, where it became much larger and was predominantly used as a weapon, not just a tool. The quick opening techniques ("flipping") were also developed in the Philippines. In contrast, the French pied du roi was primarily a folding ruler, with the knife only included in some specimens as a novelty. They were cumbersome to open and unlikely to be used for self-defense, especially since they also commonly included a metal tang at a right angle from the end of the handle to aid in measuring. There were also very similar designs to the balisong produced in England in the late 19th century, presumably also derived from the pied du roi. But like the latter, they were primarily utilitarian tools.

There are two main types of balisong construction: "sandwich construction" and "channel construction".