Dinaphthothienothiophene (DNTT) has attracted considerable attention as a next-generation material for organic thin-film transistors, replacing the conventional basic material of pentacene. Although the performance of DNTT devices is higher than that of pentacene, and has been reported numerously, a comprehensive understanding of thin-film growth is lacking. In fact, thin-film structures have long been believed to be identical to single-crystal structures. In the present study, the thickness-dependent structural evolution is revealed by means of high-resolution X-ray diffraction. This technique apparently discriminates the thin-film structure from the conventionally known bulk structure. Thus, we have revealed the thin-film phase of DNTT for the first time.