The solvent effect on the excited states of streptopolymethinemerocyanine (SPMC) was investigated using the RISM-SCF method. The magnitudes of the shifts in both the π–π* and n–π* excitation energies caused by solvation were water > methanol > acetonitrile. The analysis of the solvation structure showed that the difference in the interactions between SPMC and solvent molecules produced this order. The correlation between the chain length and the solvent effect on the excitation energy was also investigated. The results showed the excitation energy shift caused by solvation to be nearly equal regardless of the chain length.