To explore new constituents in two‐dimensional materials and to combine their best in van der Waals heterostructures, are in great demand as being unique platform to discover new physical phenomena and to design novel functionalities in interface‐based devices. Herein, PbI2 crystals as thin as few‐layers are first synthesized, particularly through a facile low‐temperature solution approach with the crystals of large size, regular shape, different thicknesses and high‐yields. As a prototypical demonstration of flexible band engineering of PbI2‐based interfacial semiconductors, these PbI2 crystals are subsequently assembled with several transition metal dichalcogenide monolayers. The photoluminescence of MoS2 is strongly enhanced in MoS2/PbI2 stacks, while a dramatic photoluminescence quenching of WS2 and WSe2 is revealed in WS2/PbI2 and WSe2/PbI2 stacks. This is attributed to the effective heterojunction formation between PbI2 and these monolayers, but type I band alignment in MoS2/PbI2 stacks where fast‐transferred charge carriers accumulate in MoS2 with high emission efficiency, and type II in WS2/PbI2 and WSe2/PbI2 stacks with separated electrons and holes suitable for light harvesting. Our results demonstrate that MoS2, WS2, WSe2 monolayers with very similar electronic structures themselves, show completely distinct light‐matter interactions when interfacing with PbI2, providing unprecedent capabilities to engineer the device performance of two‐dimensional heterostructures. 1 图 1 异质结中 PbI2引起的MoS2发光增强与WSe2发光 淬灭