5,8-dicyanopicene (CN-picene), a picene derivative that absorbs visible-light, was synthesized and its photocatalytic activity for hydrogen production via water decomposition under visible-light irradiation was evaluated. A CN-picene thin-film deposited on a quartz substrate (CNp/Q) was produced by thermal evaporation under vacuum. Hydrogen formation was observed by subjecting CNp/Q to visible-light irradiation in the presence of water. It was confirmed by mass spectrometry using D 2 O that the generated hydrogen was derived from water. Water decomposition over CNp/Q photocatalysts with different film thicknesses was also investigated; hydrogen-formation activity increased with film thickness. Furthermore, scanning electron microscopy measurements showed that CN-picene was unevenly deposited on the quartz substrate owing to its aggregation. The surface roughness of the CN-picene film increased with its thickness, which consequently increased its surface area; hence, CNp/Q catalysts with thicker films had higher water-decomposition activities than their thinner counterparts. [Display omitted] • 5,8-Dicyanopicene film deposited on quartz (CNp/Q) as an organic semiconductor. • CNp/Q enables photocatalytic water decomposition under visible-light irradiation. • CNp/Q enables photocatalytic water decomposition under solar-light irradiation. • 5,8-Dicyanopicene film thickness significantly influences hydrogen-formation rate. [ABSTRACT FROM AUTHOR]