  We describe the cohomology ring $H^*(J_2;\mathbb{F}_3)$ both as subring of
$H^*(3^{1+2}_+;\mathbb{F}_3)$ and with an abstract presentation. We also give
its Poincar\'{e} series. We use as tool a spectral sequence for the strongly
closed $3$-subgroup of $J_2$. This method might be used to compute the
cohomology of any finite simple group with a strongly closed $p$-subgroup.
