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Abstract Half‐Heusler compounds with semiconducting behavior have been developed as high‐performance thermoelectric materials for power generation. Many half‐Heusler compounds also exhibit metallic behavior without a bandgap and thus inferior thermoelectric performance. Here, taking metallic half‐Heusler MgNiSb as an example, a bandgap opening strategy is proposed by introducing the d–d orbital interactions, which enables the opening of the bandgap and the improvement of the thermoelectric performance. The width of the bandgap can be engineered by tuning the strength of the d–d orbital interactions. The conduction type and the carrier density can also be modulated in the Mg1‐xTixNiSb system. Both improved n‐type and p‐type thermoelectric properties are realized, which are much higher than that of the metallic MgNiSb. The proposed bandgap opening strategy can be employed to design and develop new half‐Heusler semiconductors for functional and energy applications.