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The wafer-scale fabrication of two-dimensional (2D) semiconductor thin films is the key to the preparation of large-area electronic devices. Although chemical vapor deposition (CVD) solves this problem to a certain extent, complex processes are required to realize the transfer of thin films from the growth substrate to the device substrate, not to mention its harsh reaction conditions. The solution-based synthesis and assembly of 2D semiconductors could realize the large-scale preparation of 2D semiconductor thin films economically. In this work, indium selenide (In₂Se₃) nanosheets with uniform sizes and thicknesses were prepared by the electrochemical intercalation of quaternary ammonium ions into bulk crystals. Layer-by-layer (LbL) assembly was used to fabricate scalable and uniform In₂Se₃ thin films by coordinating In₂Se₃ with poly(diallyldimethylammonium chloride) (PDDA). Field-effect transistors (FETs) made from a single In₂Se₃ flake and In₂Se₃ thin films showed mobilities of 12.8 cm²·V⁻¹·s⁻¹ and 0.4 cm²·V⁻¹·s⁻¹, respectively, and on/off ratios of >10³. The solution self-assembled In₂Se₃ thin films enriches the research on wafer-scale 2D semiconductor thin films for electronics and optoelectronics and has broad prospects in high-performance and large-area flexible electronics.