The ultrasonic-assisted synthesis method provides a fast, simple, and large-scale route for synthesizing desired materials under ambient conditions. In this work, we report on the facile preparation of ZnO-ZnS core-shell nanorods on a fluorine-doped tin oxide (FTO) substrate. The core-shell nanorods were synthesized by sequential nanoscale reactions involving the preparation of ZnO nanorods and conversion of the ZnO surface into a ZnS shell on the FTO substrate, using an in situ sonochemical method. The ZnO-ZnS core-shell nanorods showed improved photocurrents compared with ZnO nanorods for the water oxidation reaction. During the water oxidation reaction, the ZnS shell passivates the surface-defects of the ZnO, which results in enhanced charge separation in the ZnO nanorods and higher performance.