  The electronic structure and possible electronic orders in monolayer NbF$_4$
are investigated by density functional theory and functional renormalization
group. Because of the niobium-centered octahedra, the energy band near the
Fermi level is found to derive from the $4d_{xy}$ orbital, well separated from
the other bands. Local Coulomb interaction drives the undoped system into an
antiferromagnetic insulator. Upon suitable electron/hole doping, the system is
found to develop $d_{x^2-y^2}$-wave superconductivity with sizable transition
temperature. Therefore, the monolayer NbF$_4$ may be an exciting $4d^1$
analogue of cuprates, providing a new two-dimensional platform for high-$T_c$
superconductivity.
