Using first-principles calculations and low-energy effective model analysis, we propose that silver selenide-based ${\mathrm{RbAg}}_5{\mathrm{Se}}_3$ compound is an ideal type-II Dirac semimetal. There is a pair of Dirac points appearing at about 25.68 meV below the Fermi energy. The Dirac points existing on the $k_z$ axis are protected by $C_{4z}$ rotation symmetry in addition to $\mathcal{PT}$ symmetry. Meanwhile, the system is also characterized by a nontrivial ${\mathbb{Z}}_2$ topological invariant. The long clean Fermi arcs exist on the (010) surface, connecting the projections of the Dirac points. In addition, the Landau levels of the type-II Dirac points under magnetic field are also calculated. Our discovery not only adds one promising candidate into the type-II Dirac semimetals family but also provides an ideal platform for the study of topological phase transitions.

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• Electronic structure
• Landau levels
• Dirac semimetal
• Density functional calculations