Argyrodites, with fast lithium-ion conduction, are promising for applications in rechargeable solid-state lithium-ion batteries. We report a new compositional space of argyrodite superionic conductors, Li6–xPS5–xClBrx [0 ≤ x ≤ 0.8], with a remarkably high ionic conductivity of 24 mS/cm at 25 °C for Li5.3PS4.3ClBr0.7. In addition, the extremely low lithium migration barrier of 0.155 eV makes Li5.3PS4.3ClBr0.7 highly promising for low-temperature operation. Average and local structure analyses reveal that bromination (x > 0) leads to (i) retention of the parent Li6PS5Cl structure for a wide range of x in Li6–xPS5–xClBrx (0 ≤ x ≤ 0.7), (ii) co-occupancy of Cl–, Br–, and S2– at 4a/4d sites, and (iii) gradually increased Li+-ion dynamics, eventually yielding a “liquid-like” Li-sublattice with a flattened energy landscape when x approaches 0.7. In addition, the diversity of anion species and Li-deficiency in halogen-rich Li6–xPS5–xClBrx induce hypercoordination and coordination entropy for the Li-sublattice, also leading to enhanced Li+-ion transport in Li6–xPS5–xClBrx. This study demonstrates that mixed-anion framework can help stabilize highly conductive structures in a compositional space otherwise unstable with lower anion diversity.