Pyridinium cesium cobalt nitrate, (PyH)CsCo2(NO3)6, obtained from a nitric acid solution crystallizes in the orthorhombic space group Pnma with unit cell parameters a = 8.6905(14) Å, b = 11.9599(18) Å, c = 18.386(3) Å, V = 1911.0(5) Å3, and Z = 4. It consists of [Co(NO3)3]- layers, in which each Co2+ ion is connected with four monodentate bridging NO3-groups and one bidentate terminal NO3-group, forming a corrugated rectangular net. Magnetization and specific heat measurements show that (PyH)CsCo2(NO3)6 undergoes a long-range canted antiferromagnetic ordering in two steps at TC1 = 5.0 K and TC2 = 2.6 K. The temperature dependence of the magnetic susceptibility and the field dependence of the magnetization measured for (PyH)CsCo2(NO3)6 show that it is an Ising antiferromagnet. In support of these observations, our DFT + U + SOC calculations show that the Co2+ ions of (PyH)CsCo2(NO3)6 have an easy-axis magnetic anisotropy with preferred spin orientation along the b-axis. To a first approximation, the spin lattice of (PyH)CsCo2(NO3)6 is a weakly alternating Ising antiferromagnetic chain (J1/J2 ∼ 0.85), and these chains interact weakly (J3/J2 ∼ 0.07) to form a rectangular Ising antiferromagnetic lattice. In agreement with the prediction for a rectangular Ising antiferromagnet by Onsager, (PyH)CsCo2(NO3)6 undergoes a long-range antiferromagnetic ordering.