The optical absorption of Y 2 Ti 2 O 5 S 2 , as calculated with both hybrid DFT and QSG Ŵ +BSE methods demonstrates a slow onset due to forbidden transitions at the fundamental gap, with stronger absorption at energies above 3 eV. Furthermore, QSG Ŵ +BSE calculations that explicitly include electron-hole interaction, predict an exciton binding energy of 0.4 eV, indicating at least partial Frenkel behaviour. A tightly bound 2D exciton is predicted as a result of not only the localisation of electrons and holes in the crystallographic z direction, but also additional mixing of multiple valence bands into the excitonic wavefunction. These results indicate that to fully understand the promise of Y 2 Ti 2 O 5 S 2 as a photocatalyst, its optical and excitonic behaviour needs further consideration.