We synthesize high-quality $\mathrm{F}{\mathrm{e}}_{1-x}\mathrm{Z}{\mathrm{n}}_x\mathrm{Se}$ single crystals over a broad Zn-doping range ($x=0–0.023$) and systematically study the superconductivity of those single crystals. Accompanying the monotonic increment of Zn-doping concentration from 0.000 to 0.023, the superconducting transition temperature ($T_{\mathrm{c}}$) exhibits a pronounced nonmonotonic evolution, suggesting that the underlying mechanism governing $T_{\mathrm{c}}$ extends beyond simple impurity pair breaking. Magnetization and transport measurements confirm the bulk behavior of superconductivity and reveal enhanced scattering effects with Zn doping. The low-temperature specific-heat characteristic is consistently described by a two-gap multiband scenario composed of an isotropic $s$-wave gap and an anisotropic extended $s$-wave gap, whereas single-gap and alternative pairing symmetries fail to describe the data. The nearly unchanged relative weights of the two gap components are consistent with relatively weak interband scattering induced by Zn substitution, thereby preserving multiband superconductivity. These results demonstrate the robustness of multigap superconductivity in $\mathrm{F}{\mathrm{e}}_{1-x}\mathrm{Z}{\mathrm{n}}_x\mathrm{Se}$ and impose stringent constraints on candidate pairing mechanisms, highlighting the role of multiband electronic structure and anisotropic gap formation.

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• Impurities in superconductors
• Multiband superconductivity
• Pairing mechanisms
• Superconductivity