In recent years, the discovery of unconventional polyhydrides under high pressure, including notable instances like CaH₆, YH₉, and LaH₁₀, with superconducting critical temperature (T_c) above 200 K, has ignited considerable interest in the quest for high-temperature superconductivity in hydrogen-based materials. Recent studies have suggested the highly probable existence of hydrogen vacancies in these high-T_csuperconducting hydrides, although there is no conclusive evidence. In this study, taking niobium (Nb) hydride as a model, we showcase the observation of nonstoichiometric face-centered cubic (fcc) NbH_4-δ (δ∼0.23–0.51) at pressures ranging from 113 to 175 GPa, employing in situ high-pressure X-ray diffraction experiments in conjunction with first-principles calculations. Remarkably, our further analyses indicate that the hydrogen vacancies, along with the resulting configurational entropy, play crucial roles in stabilizing this nonstoichiometric fcc NbH_4-δ. Electrical transport measurements confirmed the superconductivity, as evidenced by zero resistance as well as suppression of T_c with applying magnetic fields, with a T_c reaching up to 34 K. Our current results not only confirm the presence of hydrogen vacancies in high-T_c hydrides, but also provide key insights into the understanding of hydrogen-vacancy-induced stability for nonstoichiometric hydrides under high pressure.