The Baeyer–Drewsen indigo synthesis (1882) is an organic reaction in which indigo is prepared from 2-nitrobenzaldehyde and acetone^[1][2] The reaction was developed by von Baeyer and Viggo Drewsen [da] in 1880 to produce the first synthetic indigo at laboratory scale. This procedure is not used at industrial scale.^[3]

The reaction is classified as an aldol condensation. As a practical route to indigo, this method was displaced by routes from aniline.^[4]

In the English literature this reaction is sometimes called Baeyer–Drewson reaction, although the author of the original paper was spelled Drewsen.