By means of spin-resolved density functional theory calculations using both atomic orbitals and plane-wave basis codes, we study the electronic and magnetic ground state of single-layer NbSe₂. We find that, for all the functionals considered, the most stable solution in this two-dimensional (2D) superconductor is the ferrimagnetic ground state with a magnetic moment of 1.09μ _B at the Nb atoms and of 0.05μ _B at the Se atoms pointing in the opposite direction. Our calculations show that the ferrimagnetic state precludes the development of charge density wave (CDW) order and their coexistence in the single-layer limit, unless graphene is considered as a substrate. The spin-resolved calculated density of states (DOS), a key fingerprint of the electronic and magnetic structure of a material, unambiguously reproduces the experimental DOS measured by scanning tunneling spectroscopy in single-layer NbSe₂. Our work sets magnetism into play in this prototypical correlated 2D material, which is crucial to understand the formation mechanisms of 2D superconductivity and CDW order.