All-inorganic perovskite solar cells (PVSCs) have attracted intensive attention owing to their tunable bandgaps and excellent photo- and thermostability, making them promising absorbers in tandem solar cells (TSCs). Herein, we develop an all-inorganic perovskite/organic TSC with a wide bandgap all-inorganic perovskite CsPbI2.1Br0.9 and a narrow bandgap organic photoactive layer (PM6:Y6) serving as top and bottom sub cells respectively. The fabricated tandem solar cell demonstrated a remarkable PCE of 18.06%, with an open-circuit voltage (VOC) of 1.89 V, short-circuit current (JSC) of 12.77 mA cm−2, and fill factor (FF) of 74.81%, which is higher than that of both single junction PVSCs (14.43%) and organic solar cells (14.01%). Moreover, benefiting from the outstanding UV and thermal stability of inorganic perovskites, all-inorganic perovskite-based tandem devices showed superior stability under light and heat, with negligible degradation after 150 h of UV irradiation, 250 h under one sun illumination and 100 h of heating at 80 °C. This work demonstrates that all-inorganic perovskites are an appropriate candidate for the fabrication of efficient and stable TSCs.