TlInSe 2 with a quasi one-dimensional chain structure and a giant Seebeck coefficient of more than 10 6 μV/K below 140°C has been investigated by means of angle-resolved photoemission spectroscopy at 50K and 280K. The obtained energy bands favorably agree with the calculated band structure and show quite noticeable dispersion in the direction normal to the chains. A rigid shift toward lower binding energies, a splitting and the formation of the mini-gap-like structures are clearly observed in the experimental electronic bands with the temperature reduced to 50K. These features are indicative of an incommensurate superlattice phase emerging in TlInSe 2 with temperature, and causing the record-breaking values of Seebeck coefficient.