Haematoxylin or hematoxylin (), also called natural black 1 or C.I. 75290, is a compound extracted from heartwood of the logwood tree (Haematoxylum campechianum) with a chemical formula of C
₁₆H
₁₄O
₆. This naturally derived dye has been used as a histologic stain, ink and as a dye in the textile and leather industry. As a dye, haematoxylin has been called Palo de Campeche, logwood extract, bluewood and blackwood. In histology, haematoxylin staining is commonly followed (counterstained), with eosin, when paired, this staining procedure is known as H&E staining, and is one of the most commonly used combinations in histology. In addition to its use in the H&E stain, haematoxylin is also a component of the Papanicolaou stain (or PAP stain) which is widely used in the study of cytology specimens.

Although the stain is commonly called haematoxylin, the active colourant is the oxidized form haematein, which forms strongly coloured complexes with certain metal ions (commonly Fe(III) and Al(III) salts). In its pure form, haematoxylin is a colourless and crystalline solid, although commercial samples are typically light to dark-brown based on the level of impurities present.