Abstract: In this study, first-principles density functional theory (DFT) calculations are employed to investigate the structural, optoelectronic, mechanical, thermodynamic, and hydrogen storage properties of XCsSiH6 (X = K, Rb). The hydrogen atoms form discrete SiH6 octahedra stabilized by alkali metal cations, confirming a stable cubic F"4" ̅"3" m symmetry upon structural optimization. Both compounds exhibit both thermal and dynamical stability. The calculated gravimetric hydrogen storage capacities are 2.94 wt% for KCsSiH6 and 2.40 wt% for RbCsSiH6. Electronic band structure analysis indicates indirect band gaps of 3.14 eV (KCsSiH6) and 3.17 eV (RbCsSiH6), with hydrogen contributing mainly to the valence band maximum (VBM) and Cs/Rb/K atoms to the conduction band minimum (CBM). Optical property analyses of the dielectric response, absorption, and reflectivity show pronounced ultraviolet activity, suggesting suitability for optoelectronic and UV-filtering applications. Both hydrides are found to be brittle yet elastically isotropic, with KCsSiH6 being slightly less stiff than RbCsSiH6. Comprehensive thermodynamic analysis demonstrates favorable variations in entropy, heat capacity, and Gibbs free energy up to 800 K, indicating the pronounced thermal stability of the investigated systems. Overall, XCsSiH6 hydrides appear to be stable semiconductors with moderate hydrogen storage capacity and desirable optical properties, suitable for various energy and electronic applications.