The synthesis of a geometrically constrained and near‐planar hexacyclic acridinium cyaninedye 9 is reported. When compared to its unlocked and non‐fluorescent monomethine cyaninedye analogue 3, this photostable dye emits in the green area of the spectrum with a remarkable quantum yield close to unity in organic solvents and above 0.5 in water. A detailed steady‐state and time‐resolved spectroscopic