Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have attracted enormous research interests and efforts towards the development of versatile electronic and optical devices, owing to their extraordinary and unique fundamental properties and remarkable prospects in nanoelectronic applications. Among the TMDs, tungsten diselenide (WSe₂) exhibits tunable ambipolar transport characteristics and superior optical properties such as high quantum efficiency. Herein, we demonstrate significant enhancement in the device performance of WSe₂ phototransistor by in situ surface functionalization with cesium carbonate (Cs₂CO₃). WSe₂ was found to be strongly doped with electrons after Cs₂CO₃ modification. The electron mobility of WSe₂ increased by almost one order of magnitude after surface functionalization with 1.6-nm-thick Cs₂CO₃ decoration. Furthermore, the photocurrent of the WSe₂-based phototransistor increased by nearly three orders of magnitude with the deposition of 1.6-nm-thick Cs₂CO₃. Characterizations by in situ photoelectron spectroscopy techniques confirmed the significant surface charge transfer occurring at the Cs₂CO₃/WSe₂ interface. Our findings coupled with the tunable nature of the surface transfer doping method establish WSe₂ as a promising candidate for future 2D materials- based optoelectronic devices.