Congo red is an organic, red or brownish-red colored, water-soluble azo dye. Its molecular formula is C32H22N6Na2O6S2. It is the sodium salt of benzidinediazo-bis-1-naphthylamine-4-sulfonic acid. The dye yields a red colloidal solution in water. Although it is a water-soluble compound, its solubility is greater in organic solvents.

Congo red is no longer used in the textile industry due to its carcinogenic properties. However, it is used in many other histology and microscopy applications.