kappa-In2Se3, a rarely reported phase of indium selenide, is synthesized here using a hot-injection method. The hexagonal nanoplates show photoresponses at positive bias, thus is considered as a potential material for photoanode utilization. In order to further improve the photoelectrocatalytic performance of kappa-In2Se3, MoS2 nanopetals are introduced by a solution-based ultrasonic method. The In2Se3/MoS2 heterojunction exhibits a significantly higher photocurrent density and higher PEC O-2 evolution amount than those of pristine In2Se3. PL test shows that the charge separation is promoted, due to numerous p-n junctions formed at the interfaces of In2Se3/MoS2. This led to more efficient utilization of photogenerated electron-hole pairs. Moreover, the charge transfer resistance is greatly reduced by the cooperative interaction of the two materials.