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Abstract A tandem (two‐step) particle swarm optimization (PSO) algorithm is implemented in the argyrodite‐based multidimensional composition space for the discovery of an optimal argyrodite composition, i.e., with the highest ionic conductivity (7.78 mS cm−1). To enhance the industrial adaptability, an elaborate pellet preparation procedure is not used. The optimal composition (Li5.5PS4.5Cl0.89Br0.61) is fine‐tuned to enhance its practical viability by incorporating oxygen in a stepwise manner. The final composition (Li5.5PS4.23O0.27Cl0.89Br0.61), which exhibits an ionic conductivity (σion) of 6.70 mS cm−1 and an activation barrier of 0.27 eV, is further characterized by analyzing both its moisture and electrochemical stability. Relative to the other compositions, the exposure of Li5.5PS4.23O0.27Cl0.89Br0.61 to a humid atmosphere results in the least amount of H2S released and a negligible change in structure. The improvement in the interfacial stability between the Li(Ni0.9Co0.05Mn0.05)O2 cathode and Li5.5PS4.23O0.27Cl0.89Br0.61 also results in greater specific capacity during fast charge/discharge. The structural and chemical features of Li5.5PS4.5Cl0.89Br0.61 and Li5.5PS4.23O0.27Cl0.89Br0.61 argyrodites are characterized using synchrotron X‐ray diffraction, Raman spectroscopy, and X‐ray photoelectron spectroscopy. This work presents a novel argyrodite composition with favorably balanced properties while providing broad insights into material discovery methodologies with applications for battery development.