When combined with transition metals with partially filled d orbitals, magnetism can be incorporated in two-dimensional materials, which greatly expands the scope for fundamental research and potential applications of these materials. Here, a first-principles study of a new two-dimensional ferromagnet VI₃ has been carried out. The structural symmetry and magnetic and electronic properties of VI₃ in its bulk and single layer forms have been confirmed and predicted, respectively. Its ferromagnetic Curie temperature is predicted to be reduced by half in its monolayer form. In addition, the cation substitution in its monolayer has also been studied, which can significantly tune the magnetism.