Ortho-phenylenediamine, often abbreviated as OPD, is an organic compound with the chemical formula C6H8N2. It is a white to light brown solid that can turn dark upon exposure to air. Ortho-phenylenediamine has several industrial applications, with one of its primary uses being in the production of dyes and pigments, particularly in the textile and paper industries.

OPD is a key intermediate in the synthesis of azo dyes, which are widely used for coloring textiles, leather, and paper products. Its ability to form a variety of colored compounds makes it valuable for achieving a wide range of hues. Additionally, ortho-phenylenediamine finds use in analytical chemistry as a reagent for the determination of various compounds. It can also be employed in certain chemical reactions to produce other valuable chemicals. Due to its potential for skin and respiratory irritation, proper safety measures should be observed when handling ortho-phenylenediamine in industrial settings.