Crystal-field perturbation is theoretically the most direct and effective method of achieving highly efficient photoluminescence from trivalent lanthanide (Ln³⁺) ions through breaking the parity-forbidden nature of their 4f-transitions. However, exerting such crystal-field perturbation remains an arduous task even in well-developed Ln³⁺-doped luminescent nanocrystals (NCs). Herein, we report crystal-field perturbation through interstitial H⁺-doping in orthorhombic-phase NaMgF₃:Ln³⁺ NCs and achieve a three-orders-of-magnitude emission amplification without a distinct lattice distortion. Mechanistic studies reveal that the interstitial H⁺ ions perturb the local charge density distribution, leading to anisotropic polarization of the F⁻ ligand, which affects the highly symmetric Ln³⁺-substituted [MgF₆]⁴⁻ octahedral clusters. This effectively alleviates the parity-forbidden selective rule to enhance the 4f–4 f radiative transition rate of the Ln³⁺ emitter and is directly corroborated by the apparent shortening of the radiative recombination lifetime. The interstitially H⁺-doped NaMgF₃:Yb/Er NCs are successfully used as bioimaging agents for real-time vascular imaging. These findings provide concrete evidence for crystal-field perturbation effects and promote the design of Ln³⁺-doped luminescent NCs with high brightness.