First-principles calculations combined with the stochastic self-consistent harmonic approximation reveal significant effects of the quantum ionic fluctuations and lattice anharmonicity on the structural stability and superconducting properties of $\mathrm{Nb}{\mathrm{H}}_3$ under high pressure. Previous theoretical predictions, which ignored ionic fluctuations and relied on the harmonic approximation, predicted the $I\overline{4}3d$ phase as the ground state between 33 and 400 GPa, with the $\mathit{Fm}\overline{3}m$ phase considered thermodynamically metastable. Harmonic calculations also placed the minimum pressure for dynamical stability of the $\mathit{Fm}\overline{3}m$ phase as high as 290 GPa. However, recent experiments identified the $\mathit{Fm}\overline{3}m$ phase at 187 GPa. Contrasting these predictions, our study reveals that the $\mathit{Fm}\overline{3}m$ phase remains dynamically stable down to at least 145 GPa—approximately 145 GPa lower than harmonic estimates. Furthermore, while the $I\overline{4}3d$ phase is dynamically stable, it exhibits higher enthalpy than the $\mathit{Fm}\overline{3}m$ phase at 187 GPa, aligning with the experimental observations. We systematically investigate the structural, vibrational, and superconducting properties of the $\mathit{Fm}\overline{3}m \mathrm{Nb}{\mathrm{H}}_3$ under pressures ranging from 100 to 300 GPa, uncovering substantial modifications induced by the quantum and anharmonic effects. The calculated superconducting critical temperature ($T_{\mathrm{c}}$) from the McMillan equation (with ${\mu }^{*}=0.15$) for $\mathit{Fm}\overline{3}m \mathrm{Nb}{\mathrm{H}}_3$ at 187 GPa is 44 K, close to the measured value. These findings highlight the crucial role of quantum anharmonic effects in shaping the structural and vibrational properties of $\mathrm{Nb}{\mathrm{H}}_3$ at high pressures.

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• Density of states
• Electronic structure
• Fermi surface
• Phase transitions by order
• Phonons
• Quantum phase transitions
• Structural properties
• Superconducting gap
• Superconductivity