<p>Solution-processable two-dimensional semiconductors could be used for the cost-effective fabrication of large-area electronic devices. However, compared with two-dimensional materials made using high-temperature chemical vapour deposition methods, they exhibit much lower electrical performance. Here we report the solution-processable fabrication of wafer-scale two-dimensional layered copper indium selenide (CuIn₅Se₈) semiconductors with high mobility values. The air-stable solution-based ink formulation and film assembly allow four-inch CuIn₅Se₈ thin films to be deposited in the ambient air environment. By elucidating the impact of a reversible water adsorption process on electrical performance, a moderate annealing step is developed to eliminate surface water molecules. The resulting CuIn₅Se₈ transistors exhibit an average electron mobility of 155 cm² V⁻¹ s⁻¹ and an on/off ratio of 10⁷ with a small current hysteresis. Furthermore, the robust transistor operation and small device-to-device variation enable the integration of over 100 transistors into a prototypical microprocessor that can process digital signals at a frequency of 2.2 kHz.</p>