The triboelectric effect (also known as triboelectric charging) is a type of contact electrification on which certain materials become electrically charged after they are separated from a different material with which they were in contact. Rubbing the two materials with each other increases the contact between their surfaces, and hence the triboelectric effect. Rubbing glass with fur for example, or a plastic comb through the hair, can build up triboelectricity. Most everyday static electricity is triboelectric. The polarity and strength of the charges produced differ according to the materials, surface roughness, temperature, strain, and other properties.

The triboelectric effect is very unpredictable, and only broad generalizations can be made. Amber, for example, can acquire an electric charge by contact and separation (or friction) with a material like wool. This property was first recorded by Thales of Miletus. The word "electricity" is derived from William Gilbert's initial coinage, "electra", which originates in the Greek word for amber, ēlektron. The prefix tribo- (Greek for ‘rub’) refers to ‘friction’, as in tribology. Other examples of materials that can acquire a significant charge when rubbed together include glass rubbed with silk, and hard rubber rubbed with fur.

A very familiar example could be the rubbing of a plastic pen on a sleeve of almost any typical material like cotton, wool, polyester, or blended fabric used in modern clothing. Such an electrified pen will attract and pick up pieces of paper less than a square centimeter when the pen approaches. Also, such a pen will repel a similarly electrified pen. This repulsion is readily detectable in the sensitive setup of hanging both pens on threads and setting them nearby one another. Such experiments led to the theory of two types of quantifiable electric charge, one being effectively the negative of the other, with a simple sum respecting signs giving the total charge. The electrostatic attraction of the charged plastic pen to neutral uncharged pieces of paper (for example) is due to temporary charge separation (electric polarisation or dipole moment) of electric charges within the paper (or perhaps alignments of permanent molecular or atomic electric dipoles). A net force then arises as the slightly nearer charges of the dipole get attracted more strongly in the nonuniform field from the pen which diminishes with distance. In a uniform electric field, for example inside parallel capacitor plates, temporary polarisation would occur in the small pieces of paper but with zero net attraction.

The triboelectric effect is now considered to be related to the phenomenon of adhesion, where two materials composed of different molecules tend to stick together because of attraction between the different molecules. While adhesion is not a chemical bond between atoms, there is an exchange of electrons between the different types of molecules, resulting in an electrostatic attraction between the molecules that holds them together. Physical separation of materials that are adhered together results in friction between the materials. Because the electron transfer between molecules in the different materials is not immediately reversible, the excess electrons in one type of molecule remain left behind, while a deficit of electrons occurs in the other. Thus, a material can develop a positive or negative charge (see also static electricity) that dissipates after the materials separate.

The mechanisms of triboelectrification (or contact-electrification) have been debated for many years, with possible mechanisms including electron transfer, ion transfer or the material's species transfer. Recent studies in 2018 using Kelvin probe microscopy and triboelectric nanogenerators revealed that electron transfer is the dominant mechanism for triboelectrification between solid and solid. The work function model can be used to explain electron transfer between a metal and a dielectric. The surface states model can be used to explain electron transfer between two dielectrics. For a general case, since triboelectrification occurs for any material, a generic model has been proposed by Wang, in which the electron transfer is caused by a strong electron cloud overlap between two atoms for the lowered interatomic potential barrier by shortening the bonding length. Based on the model, the effects of temperature and photo excitation on the triboelectrification were investigated. Such model can be further extended to the cases of liquid-solid, liquid-liquid and even gas-liquid.