Substituted indigo derivatives undergo photoisomerization of the central double bond if both nitrogen atoms are functionalized. Indigo itself however does not photoisomerize because of a competing and highly efficient excited‐state proton transfer. In this work, we show that also mono‐arylated indigo undergoes photoisomerization despite still possessing one nitrogen‐bound proton and the likely presence of a competing intramolecular excited‐state proton transfer. The two different isomers exhibit strongly different absorptions and therefore can be distinguished by the naked eye. Different to diaryl‐substituted indigo, thermal decay of the metastable cis isomers of mono‐arylated derivatives can be greatly accelerated by small amounts of water (by more than a factor of 300 for measured half‐lives). Such tunability is of high interest for applications that require quick and autonomous switching‐off of photoresponsive trigger units.