The present invention provides a heterojunction photocatalyst having higher photocatalytic activity than that of a conventional heterojunction photocatalyst. Further, the present invention provides a photocatalyst composite having the heterojunction photocatalyst on a substrate, a method for producing the heterojunction photocatalyst, and a method for producing hydrogen using the heterojunction photocatalyst or the photocatalyst composite The het junction photocatalyst of the present invention has a solid mediator between a hydrogen-evolution photocatalyst and an oxygen-evolution photocatalyst, and the solid mediator is selectively joined to an electrons collecting surface of the oxygen-evolution photocatalyst.