We demonstrate a monolithic perovskite/CIGS tandem solar cell with a certified power conversion efficiency (PCE) of 24.2%. The tandem solar cell still exhibits photocurrent mismatch between the subcells; thus optical simulations are used to determine the optimal device stack. Results reveal a high optical potential with the optimized device reaching a short-circuit current density of 19.9 mA cm–2 and 32% PCE based on semiempirical material properties. To evaluate its energy yield, we first determine the CIGS temperature coefficient, which is at −0.38% K–1 notably higher than the one from the perovskite subcell (−0.22% K–1), favoring perovskite in the field operation at elevated cell temperatures. Both single-junction cells, however, are significantly outperformed by the combined tandem device. The enhancement in energy output is more than 50% in the case of CIGS single-junction device. The results demonstrate the high potential of perovskite/CIGS tandem solar cells, for which we describe optical guidelines toward 30% PCE.