Room-temperature ferromagnetism in monolayer vanadium diselenide (VSe2) on graphite is object of a controversial debate. Herein, we unveil the contribution from extrinsic factors to the magnetic properties of monolayer VSe2 by means of density functional theory. Specifically, we demonstrate that either intrinsic defects or adsorption of molecules enhance ferromagnetic interactions. The expansion of the VSe2 lattice increases the magnetic moment on vanadium ions, whereas both compression and out-of-plane distortion withdraw magnetic moments. The exchange interactions between vanadium ions and magnetic defects (vacancies and iron impurities) in surface and subsurface layers of the substrate are able to turn the unstable two-dimensional (2D) ferromagnetism into stable three-dimensional (3D) ferromagnetism. The combination of effects related to chemisorption, substrate-induced distortion and …