In the crystal structure of the title salt, C4H6N3O(+)·HSeO3 (-), systematic name 6-amino-2-methyl-idene-2,3-di-hydro-pyrim-idin-1-ium hydrogen selenite, the hydrogenselenite anions and the cytosinium cations are linked via N-H⋯O, N-H⋯Se, O-H⋯O, O-H··Se and C-H⋯O hydrogen bonds, forming a three-dimensional framework.