  We report on the magnetic properties of CsCo$_2$Se$_2$ with ThCr$_2$Si$_2$
structure, which we have characterized through a series of magnetization and
neutron diffraction measurements. We find that CsCo$_2$Se2$_2$ undergoes a
phase transition to an antiferromagnetically ordered state with a N\'eel
temperature of $T_{\rm N} \approx$ 66 K. The nearest neighbour interactions are
ferromagnetic as observed by the positive Curie-Weiss temperature of $\Theta
\approx$ 51.0 K. We find that the magnetic structure of CsCo$_2$Se$_2$ consists
of ferromagnetic sheets, which are stacked antiferromagnetically along the
tetragonal \textit{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)$\mu_{\rm Bohr}$/Co. Already in comparably small
magnetic fields of $\mu_0 H_{MM}$(5K) $\approx$ 0.3 T, we observe a
metamagnetic transition that can be attributed to spin-rearrangements of
CsCo$_2$Se$_2$, with the moments fully ferromagnetically saturated in a
magnetic field of $\mu_0 H_{\rm FM}$(5K) $\approx$ 6.4 T. We discuss the entire
experimentally deduced magnetic phase diagram for CsCo$_2$Se$_2$ with respect
to its unconventionally weak magnetic coupling. Our study characterizes
CsCo$_2$Se$_2$, which is chemically and electronically posed closely to the
$A_xFe_{2-y}Se_2$ superconductors, as a host of versatile magnetic
interactions.
