A substantial energy gap of charge excitations induced by strong correlations is the characteristic feature of Mott insulators. We study how the Mott gap is affected by the long-range antiferromagnetic order. Our key finding is that the Mott gap is enhanced by the magnetic ordering: a magnetic blue-shift (MBS) occurs. We establish this important and general effect in a three-dimensional Hubbard model, the paradigm for strongly correlated systems, without any spin-orbit term. The MBS is thus of exchange origin and can increase the Mott gap by approximately $70%$ as the temperature decreases from the ordering temperature to zero. The coupling between spin and charge degrees of freedom bears the potential to enable spin-to-charge conversion in Mott systems on extreme time-scales determined by the exchange only, since spin-orbit mediated transfer of angular momentum is not involved in the process. In view of spintronic and magnonic applications, we show that the magnetic contribution to the band-gap blue-shift observed in the optical conductivity of $alpha$-MnTe is correctly interpreted as the MBS of a Mott gap.
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