This work reports for the first time a highly efficient single‐crystal cesium tin triiodide (CsSnI3) perovskite nanowire solar cell. With a perfect lattice structure, low carrier trap density (≈5 × 1010 cm−3), long carrier lifetime (46.7 ns), and excellent carrier mobility (>600 cm2 V−1 s−1), single‐crystal CsSnI3 perovskite nanowires enable a very attractive feature for flexible perovskite photovoltaics to power active micro‐scale electronic devices. Using CsSnI3 single‐crystal nanowire in conjunction with highly conductive wide bandgap semiconductors as front‐surface‐field layers, an unprecedented efficiency of 11.7% under AM 1.5G illumination is achieved. This work demonstrates the feasibility of all‐inorganic tin‐based perovskite solar cells via crystallinity and device‐structure improvement for the high‐performance, and thus paves the way for the energy supply to flexible wearable devices in the future. Efficient solar cell is achieved based on single‐crystal cesium tin triiodide nanowires with a long carrier lifetime and a large carrier mobility. The unique front‐surface‐field device structure of single‐crystal nanowire solar cells is proven to be an effective means of enhancing the carrier extraction and effectively boosting efficiency.