Advanced nonlinear optical (NLO) materials, such as metal chalcogenides, black phosphorus, and perovskites, have encountered challenges arising from their inherent low chemical and thermal stability. The desirable attributes of chemical and thermal stability are exhibited by promising metal oxides within NLO applications, whereas the NLO response of metal oxides has typically shown modest strength. In this study, we report the striking enhancement of NLO absorption in W18O49 achieved through the introduction of defects via Cu doping. Following a 4-hour doping treatment, the nonlinear absorption coefficients (βeff) of Cu-doped W18O49 exhibit remarkable values of (3.36 ± 0.11) × 103 cm GW−1 and (5.76 ± 0.33) × 103 cm GW−1 upon laser excitation at 515 and 800 nm, respectively. These values are approximately 10 and 32 times higher than those measured in the pristine samples. This performance surpasses that of various other metal oxides and stands on par with the optimal tungsten/molybdenum-based chalcogenide counterparts. Notably, our investigation reveals a direct correlation between higher levels of Cu doping and an augmented βeff. Our investigations using X-ray photoelectron spectroscopy, Raman spectroscopy, ultraviolet photoelectron spectroscopy, electron paramagnetic resonance, and density functional theory calculations reveal that Cu doping induces the creation of oxygen vacancies, precipitating a concurrent reduction in bandgap and the emergence of in-gap defect states. Transient transmittance difference spectra highlight the effective role played by in-gap defect states as excitation pathways. Consequently, a greater abundance of these in-gap states corresponds to heightened optical nonlinearity. © 2024 Elsevier Ltd.