This communication describes a facile hot-injection method for the synthesis of novel micrometer-size formamidinium lead bromide perovskite (FAPbBr3) nanosheets of monolayer thickness (around 2.4 nm). Due to the strong quantum confinement effect, the as-synthesized large-sized perovskite nanosheets displayed an unprecedented extremely blue-shifted first sharp excitonic absorption peak at 435 nm and an emission peak at 437 nm, as compared to that of bulk FAPbBr3 crystals at 551 nm.