Solution-processable, single-crystalline perovskite nanowires are ideal candidates for developing low-cost photodetectors, but their detectivities are limited due to a high level of unintentional defects. Through the surface-initiated solution-growth method, we fabricated high-quality, single-crystalline, defects-suppressed ${\mathrm{MAPbI}}_3$ nanowires, which possess atomically smooth side surfaces with a surface roughness of 0.27 nm, corresponding to a carrier lifetime of 112.9 ns. By forming ohmic ${\mathrm{MAPbI}}_3/\mathrm{Au}$ contacts through the dry contact method, high-performance metal–semiconductor–metal photodetectors have been demonstrated with a record large linear dynamic range of 157 dB along with a record high detectivity of $1.2\times {10}^{14}\mathrm{Jones}$ at an illumination power density of $5.5\mathrm{nW}/{\mathrm{cm}}^2$. Such superior photodetector performance metrics are attributed to, first, the defects-suppressed property of the as-grown ${\mathrm{MAPbI}}_3$ nanowires, which leads to a quite low noise current in the dark, and second, the ohmic contact between ${\mathrm{MAPbI}}_3$ and Au interfaces, which gives rise to an improved responsivity compared with the Schottky contact counterpart. The realized high-performance ${\mathrm{MAPbI}}_3$ nanowire photodetector advances the development of low-cost photodetectors and has potential applications in weak-signal photodetection.