Transport measurements on thin films of the prototypical altermagnet MnTe have yielded conflicting reports including the anomalous Hall effect (AHE) with opposite signs and resistivity independent of thickness. In this work, we resolve these discrepancies by disentangling the bulk and surface contributions to the AHE for various crystal terminations. Using first-principles calculations and symmetry-based effective models, we demonstrate that, while the bulk hosts a characteristic $g$-wave Fermi surface, metallic surface states within the bulk gap acquire ferromagnetlike spin polarization and dominate the AHE at experimentally relevant Fermi levels. Although the surface magnetization direction is determined by the terminating spin sublattice, the sign of the surface AHE follows the bulk Néel order for any fixed termination. Both bulk and surface contributions share a common symmetry origin with a small but finite out-of-plane orbital magnetization, which reverses together with the AHE upon Néel-order reversal. Furthermore, realistic interfacial chemistry sets the magnitude and overall sign of the surface AHE—a Te capping layer reverses it relative to an InP substrate interface—while the locking to the bulk Néel order persists for each interface. Our results establish how interface design can be used to control magnetotransport in altermagnetic thin films.

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• Anomalous Hall effect
• Surface states
• Altermagnets
• First-principles calculations