We investigate a quantum-well sample containing a monolayer of MnTe in its center. The exciton Zeeman splitting is measured as a function of temperature by polarization-dependent photoluminescence excitation experiments. The dependence of the inverse Zeeman splitting shows a kink at a critical temperature of 50 K. Simultaneously, a peak in the linewidth of the e1–hh1 transition is observed at the same temperature. The critical temperature agrees very well with the predicted Nèel temperature obtained within the mean-field approximation for a single monolayer MnTe. After annealing the sample, the Zeeman splitting drastically increases and no critical phenomena are observed anymore.