Two-dimensional (2D) ferromagnetic (FM) half-metallic materials have attracted intensive attention due to their unique electronic and magnetic properties and potential applications in spintronic devices. In this study, we predicted a stable 2D half-metallic material monolayer CrSiSe4 using first-principles density functional theory. The structure, electronic and magnetic properties were systematically studied. The calculations show that the monolayer CrSiSe4 is a dynamically stable FM half-metallic material. The spin-dependent transport properties and the Curie temperature up to 239 K are demonstrated. The spin band gap of monolayer CrSiSe4 was about 0.83 eV by the the Heyd–Scuseria–Ernzerhof function calculation. The magnetic anisotropy energy of each Cr atom in the monolayer of CrSiSe4 is
−
552.3
μ
eV. When the applied biaxial tensile strain is greater than 2%, monolayer CrSiSe4 spin-up conduction band and valence band will show a band gap at the Fermi level, and the electronic properties change from a half-metal to a semiconductor. Thus, the monolayer CrSiSe4 can provide an ideal candidate material for exploring 2D magnetic and spintronics experiments.