Thiazine-indigos are novel hydrogen-bonded pigments developed recently by Clariant that exhibit a variety of shades from yellow to red. Among these, a commercially available chlorinated derivative (THI) gives a color of vivid red, although it is only pale yellow in solution. For this reason, electronic characterization of THI has been carried out on the basis of the crystal structure. There are chains of intermolecular hydrogen bonds between the N-H group of one molecule and the 0 atom of the neighboring one along the b-axis, forming a two-dimensional hydrogen bond network. The direction of the transition dipole points along the long molecular axis, and the transition dipoles are arranged in "head-to-tail" fashion along the (b,-c) diagonal direction. The large bathochromic shift upon crystallization can be mostly attributed to excitonic interactions between transition dipoles arranged diagonally along the stacking axis as well as those in head-to-tail fashion on the molecular plane.