MoS2/ZnIn2S4 nanocomposite was prepared via photoreduction of (NH4)(2)MoS4 in the presence of hexagonal ZnIn2S4. The as-obtained MoS2/ZnIn2S4 nanocomposite showed superior activity for acceptorless photocatalytic semi-dehydrogenation of 1,2,3,4-tetrahydroisoquinoline to produce 3,4-dihydroisoquinoline, with quantitative hydrogen evolved. In addition to MoS2/ZnIn2S4, a series of MS/ZnIn2S4 nanocomposites (MS = PtS and NiS) were also found to be active for this reaction, indicating that ZnIn2S4-based nanocomposites are effective photocatalysts for acceptorless semi-dehydrogenation of 1,2,3,4-tetrahydroisoquinoline to produce 3,4-dihydroisoquinoline. This study not only provides an efficient, green and cost effective strategy to produce 3,4-dihydroisoquinoline, but also highlights the great potential of semiconductor-based photocatalysis for light-driven organic syntheses.