Summary: The electronic, optical, photocatalytic, and thermoelectric properties of inorganic halide cubic perovskite AGeI2Br (A = K, Rb, Cs) are investigated using the density functional theory, in order to use these compounds in different fields. The aim of our investigation is to bring a prediction of the free lead halide mixed compound which is still not synthesized until now, and show the potential use of these materials in the solar field. The results show that the studied compounds exhibit remarkable properties like their high absorption coefficient (over 105 cm−1) in the visible range, but the only inconvenience of these materials is that they have slightly low bandgap energy. For this purpose, the tensile effect is investigated to recognize its effect on the bandgap values and how it can be helpful to be more useful for photovoltaic application. Besides, the mechanism of photocatalytic water‐splitting was studied based on the calculated bandgap energy, relative ratios of effective mass, and the valence band and the conduction band edge potential. [ABSTRACT FROM AUTHOR]