4-Hydroxy-3-nitrobenzaldehyde is an organic compound featuring a benzaldehyde core substituted with a hydroxyl group at the 4-position and a nitro group at the 3-position. It is a yellow to orange crystalline powder with a molecular formula of C7H5NO4. This compound serves as a key intermediate in the synthesis of various dyes and pigments. Its structure, containing both an aldehyde and a phenolic hydroxyl group, allows for further chemical modifications, such as condensation reactions to form azomethine (Schiff base) dyes or azo dyes, which are important for colorants in textiles and other materials. It is also utilized in the preparation of some pharmaceutical intermediates and organic light-emitting materials. As a versatile building block, it is commercially available for research and industrial applications. Standard safety precautions for handling nitroaromatic compounds and aldehydes should be observed.