2D lead iodide (PbI2) is attracting great interest due to its great potential in the application of UV photodetectors. In this work, a facile solution‐based method is developed to synthesize ultraflat PbI2 nanoflakes for high‐performance UV photodetectors. By maintaining at proximate room temperature and adding an evaporation suppression solvent for slow‐rate crystal growth, high‐quality PbI2 nanoflakes with an ultraflat surface are obtained. The UV photodetectors based on 2D PbI2 nanoflakes exhibit a high photoresponsivity of 0.51 A W−1, a high detectivity of 4.0 × 1010 Jones, a high external quantum efficiency (EQE) of 168.9%, and a rapid response speed including a rise time of 14.1 ms and a decay time of 31.0 ms. The balanced and excellent photodetector performance of these devices paves the road for practical UV photodetection based on 2D PbI2 nanoflakes. The growth of ultraflat PbI2 nanoflakes is realized by solvent evaporation suppression. Optimized surface topography endows superior performance to PbI2‐based photodetectors, including a high responsivity of 0.51 A W−1, a high detectivity of 4.0 × 1010 Jones, a high external quantum efficiency of 168.9%, a rapid rise time of 14.1 ms, and a decay time of 31.0 ms.