<p>Halides are promising solid electrolytes due to their high ionic conductivity and high oxidation potential. Here we report a superionic chloride material, NaTaCl₆, which exhibits a high ionic conductivity of 3.3 mS cm⁻¹ at 27 °C, being two-orders of magnitude higher than that of NaNbCl₆ (0.01 mS cm⁻¹). The considerably higher conductivity exhibited by NaTaCl₆ vs. NaNbCl₆ arises from the more facile rotational/reorientational dynamics of the [TaCl₆] polyanions in comparison to the [NbCl₆] anions. [TaCl₆] polyanion rotation is readily activated while [NbCl₆] polyanion reorientation is hindered at room temperature but can be turned on as the temperature increases or under prolonged mechanical milling. The higher degree of structural disorder exhibited by NaTaCl₆ compared to NaNbCl₆—likely attributed to its greater mechanical and phonon softness—is found to contribute to the more pronounced [TaCl₆] anion rotation. Anion rotation is coupled with, and facilitates, macroscopic Na⁺-ion diffusion. As a result, enhanced rotational dynamics are directly correlated with the higher Na⁺-ion conductivity observed in NaTaCl₆. The high ionic conductivity, combined with its electrochemical stability against positive electrode materials, enables good rate capability and long-term cycling performance in solid-state cells. These findings provide insights into ion transport mechanism in the newly emerging halide solid electrolytes.</p>