[en] 2D perovskites have attracted wide attention for optoelectronic applications because of their unique layer structure and tunable outstanding optical/electrical properties. In addition, 2D Cs${}_3$Bi${}_2$Br${}_9$ nanoflakes possess large effective atomic number, high resistivity, high density as well as excellent stability, rendering it a promising material for X-ray detection. Nevertheless, it is full of challenges to synthesize 2D Cs${}_3$Bi${}_2$Br${}_9$ nanoflakes by conventional inversion temperature crystallization (ITC) strategy due to the existence of Br${}^{-<!--\; ???\; -->}$ vacancies in the Cs${}_3$Bi${}_2$Br${}_9$ crystal nucleus. Herein, an Ag${}^{+}$ assisted ITC (SAITC) strategy to grow 2D Cs${}_3$Bi${}_2$Br${}_9$ nanoflakes is proposed. The synthesis mechanism revealed by both experiments and theoretical calculations can be mainly ascribed to the passivated Br${}^{-<!--\; ???\; -->}$ vacancies and enhanced structure stability by adding Ag${}^{+}$ which can effectively prevent the oxidation of 2D Cs${}_3$Bi${}_2$Br${}_9$ nanoflakes from growth of hybrid crystals. The synthesized high-crystallinity 2D Cs${}_3$Bi${}_2$Br${}_9$ nanoflakes possess direct bandgap characteristic, and the mobility lifetime can reach 9.8 × 10${}^{-<!--\; ???\; -->4}$ cm${}^2$ V${}^{-<!--\; ???\; -->1}$. Excitingly, the fabricated device based on 2D Cs${}_3$Bi${}_2$Br${}_9$ nanoflakes demonstrates ultrahigh sensitivity of detecting X-ray (1.9 CGy${}_{air}$${}^{-<!--\; ???\; -->1}$ cm${}^{-<!--\; ???\; -->2}$) at very low driven voltage (0.5 V) due to the photoconductive gain mechanism. The 2D Cs${}_3$Bi${}_2$Br${}_9$ nanoflakes synthesized by SAITC method have great potential for developing highly sensitive optoelectronic devices. (© 2023 Wiley‐VCH GmbH)