The square-kagome lattice (SKL) antiferromagnet, composed of two-dimensional corner-sharing triangles, provides a prominent platform for studying frustrated magnetism. However, material realizations of the SKL remain scarce. Here, we report the single-crystal growth, structural characterization, magnetic and electric properties of ${\mathrm{KCu}}_7{\mathrm{TeO}}_4{\left({\mathrm{SO}}_4\right)}_5\mathrm{Cl}$, a nabokoite compound featuring a distorted and decorated SKL. Weak anomalies at temperature near 4 K are observed in both the magnetization and the specific heat data, indicating the onset of a magnetic transition. The observation of the ${}^{35}\mathrm{Cl}$ nuclear magnetic resonance line splits below 4.5 K further confirms the formation of the long-range-order antiferromagnetism. Magnetic susceptibility data reveal nearly isotropic Curie-Weiss temperatures ($\sim -145\mathrm{K}$) and $g$ factors ($\sim 2.4$) for both in-plane and out-of-plane magnetic fields. Moreover, we observe two successive ferroelectric transitions at $T_{\mathrm{FE}}1\sim 30\mathrm{K}$ and $T_{\mathrm{FE}}2\sim 27\mathrm{K}$, driven by inversion symmetry breaking, most likely associated with distortions in the ${\mathrm{Cu}2\mathrm{O}}_4{\mathrm{Cl}}_1$ pyramids and the adjacent ${\mathrm{SO}}_4$ tetrahedra. These results suggest that a three-dimensional model incorporating interlayer couplings via decorating Cu2 sites is essential for capturing the magnetic and electric behaviors in ${\mathrm{KCu}}_7{\mathrm{TeO}}_4{\left({\mathrm{SO}}_4\right)}_5\mathrm{Cl}$.

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• Ferroelectricity
• Magnetism
• Structural properties
• Magnetization measurements
• Nuclear magnetic resonance
• Specific heat measurements
• X-ray diffraction