Single-phase two-dimensional (2D) indium monoselenide (γ-InSe) film is successfully grown via solid phase epitaxy in the molecular beam epitaxy (MBE) system. Having high electron mobility and high photoresponsivity, ultrathin 2D γ-InSe semiconductors are attractive for future field-effect transistor and optoelectronic devices. However, growing single-phase γ-InSe film is a challenge due to the polymorphic nature of indium selenide (γ-InSe, α-In₂Se₃, β-In₂Se₃, γ-In₂Se₃, etc.). In this work, the 2D α-In₂Se₃ film was first grown on a sapphire substrate by MBE. Then, the high In/Se ratio sources were deposited on the α-In₂Se₃ surface, and an γ-InSe crystal emerged via solid-phase epitaxy. After 50 min of deposition, the initially 2D α-In₂Se₃ phase was also transformed into a 2D γ-InSe crystal. The phase transition from 2D α-In₂Se₃ to γ-InSe was confirmed by Raman, XRD, and TEM analysis. The structural ordering of 2D γ-InSe film was characterized by synchrotron-based grazing-incidence wide-angle X-ray scattering (GIWAXS).