Pyrite-type mineral hauerite MnS₂ is a magnetic insulator with localized S=5/2 moments. Below 48 K, it develops a collinear antiferromagnetic order, accompanied by unit-cell doubling and breaking the inversion symmetry. We demonstrate that the latter gives rise to a ferroelectric polarization of up to 360 μC/m² rendering MnS₂ a type-II multiferroic. Microscopically, the electronic polarization originates from S₂ dimers that acquire a dipole moment induced by the frustrated magnetic ordering of neighboring Mn sites. We show that time-reversal symmetry leaves ferroelectric domains intact, and that domain switching necessitates overcoming a high energy barrier set by the magnetic exchange energy scale.