TiO₂–ZnS core–shell composite nanorods were synthesized by using ZnO as a sacrificial shell layer in a hydrothermal reaction. ZnO thin films of different thicknesses were sputter-deposited onto the surfaces of TiO₂ nanorods as templates for hydrothermally synthesizing TiO₂–ZnS core–shell nanorods. Structural analysis revealed that crystalline TiO₂–ZnS composite nanorods were formed without any residual ZnO phase after hydrothermal sulfidation in the composite nanorods. The thickness of the ZnO sacrificial shell layer affected the surface morphology and sulfur-related surface defect density in hydrothermally grown ZnS crystallites of TiO₂–ZnS composite nanorods. Due to the distinctive core–shell heterostructure and the heterojunction between the TiO₂ core and the ZnS shell, TiO₂–ZnS core–shell nanorods exhibited ethanol gas-sensing performance superior to that of pristine TiO₂ nanorods. An optimal ZnO sacrificial shell layer thickness of approximately 60 nm was found to enable the synthesis of TiO₂–ZnS composite nanorods with satisfactory gas-sensing performance through sulfidation. The results demonstrated that hydrothermally derived TiO₂–ZnS core–shell composite nanorods with a sputter-deposited ZnO sacrificial shell layer are promising for applications in gas sensors.