To develop a stable and reliable two-dimensional (2D) tungsten disulfide (WS2)-based photodetector (PD), it is essential to address the issue of interfacial defects that are unavoidably formed at an interface between WS2 and metal contact, as such defects can markedly deteriorate photo-response characteristics. In this work, this drawback is mitigated by adopting a facile technique for passivating a WS2 surface with an ultrathin TiO2 film. The TiO2 interlayer is deposited on the 2D-WS2 surface via twenty cycles of atomic layer deposition (ALD) prior to proceeding with photolithography and contact metal deposition. Advanced characterizations reveal that TiO2/WS2 PD exhibits enhanced photo-response compared to bare WS2. Much higher photo-responsivity (~10 times higher at 1 mW/cm2) and faster recovery (~124 times faster at 0.1 V) is obtained from TiO2/WS2 PDs relative to bare WS2 PDs. The mechanism underlying the enhanced PD performance is faithfully demonstrated. The computational density functional theory (DFT) using Hyed-Scuseria-Ernzerhof (HSE) approach demonstrates the significant role of TiO2/WS2 interface in facilitating the charge transfer, and improving the PD performance compared to the bare device. This approach paves the way for developing reliable and high-performance 2D WS2-based optoelectronic devices.