KSiH₃ exhibits 4.1 wt% experimental hydrogen storage capacity and shows reversibility under moderate conditions, which provides fresh impetus to the search for other complex hydrides in the K–Si–H system. Here, we reproduce the stable Fmm phase of K₂SiH₆ and uncover two denser phases, space groups Pm1 and P6₃mc at ambient pressure, by means of first-principles structure searches. We note that Pm1-K₂SiH₆ has a high hydrogen content of 5.4 wt% and a volumetric density of 88.3 g L⁻¹. Further calculations suggest a favorable dehydrogenation temperature T_des of −20.1/55.8 °C with decomposition into KSi + K + H₂. The higher hydrogen density and appropriate dehydrogenation temperature indicate that K₂SiH₆ is a promising hydrogen storage material, and our results provide helpful and clear guidance for further experimental studies. We found three further potential hydrogen storage materials stable at high pressure: K₂SiH₈, KSiH₇ and KSiH₈. These results suggest the need for further investigations into hydrogen storage materials among such ternary hydrides at high pressure.