We report on the magnetic properties of $CsCo_2Se_2$ with $ThCr_2Si_2$ structure, which we have characterized through a series of magnetization and neutron diffraction measurements. We find that $CsCo_2Se_2$ undergoes a phase transition to an antiferromagnetically ordered state with a Néel temperature of $T_N ≈ 66 K$. The nearest neighbour interactions are ferromagnetic as observed by the positive Curie-Weiss temperature of $Θ ≈ 51.0 K$. We find that the magnetic structure of $CsCo_2Se_2$ consists of ferromagnetic sheets, which are stacked antiferromagnetically along the tetragonal c-axis, generally referred to as A-type antiferromagnetic order. The observed magnitude of the ordered magnetic moment at T = 1.5 K is found to be only 0.20(1)$μ_{Bohr}$ / Co. Already in comparably small magnetic fields of $μ_0H _{MM}(5~K) ≈ 0.3 T$, we observe a metamagnetic transition that can be attributed to spin-rearrangements of $CsCo_2Se_2$, with the moments fully ferromagnetically saturated in a magnetic field of $μ_0 H_{FM}(5~K) ≈ 6.4 T$. We discuss the entire experimentally deduced magnetic phase diagram for $CsCo_2Se_2$ with respect to its unconventionally weak magnetic coupling. Our study characterizes $CsCo_2Se_2$, which is chemically and electronically posed closely to the $A_xFe_{2-y}Se_2$ superconductors, as a host of versatile magnetic interactions.