Magnetic two-dimensional materials have attracted considerable attention for their significant potential application in spintronics. Here, we systematically study the electronic structures and magnetic properties of a 3d transition metal doped InSe monolayer based on density functional theory (DFT). Our results show that InSe monolayer can be turned into a half-metal when the Ti, Cr, or Ni atom is doped. Further calculations indicate that the Cr-InSe monolayer possesses a robust ferromagnetic ground state due to the effective p–d exchange. The predicted Curie temperature of Cr-InSe is above the room temperature, showing a powerful potential in spintronics. Application in the spin valve is also explored using quantum transport simulations. Our results indicate that the magnetoresistance of a Cr-InSe spin valve attains 100% due to the half-metallic characteristics. These findings may pave the way for designing 2D nano-devices for future spin transport applications.