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Abstract Bi₂SeO₂is a promisingn‐type semiconductor to pair withp‐type BiCuSeO in a thermoelectric (TE) device. The TE figure of meritzTand, therefore, the device efficiency must be optimized by tuning the carrier concentration. However, electron concentrations in self‐dopedn‐type Bi₂SeO₂span several orders of magnitude, even in samples with the same nominal compositions. Such unsystematic variations in the electron concentration have a thermodynamic origin related to the variations in native defect concentrations. In this study, first‐principles calculations are used to show that the selenium vacancy, which is the source ofn‐type conductivity in Bi₂SeO₂, varies by 1–2 orders of magnitude depending on the thermodynamic conditions. It is predicted that the electron concentration can be enhanced by synthesizing under more Se‐poor conditions and/or at higher solid‐state reaction temperatures (T_SSR), which promote the formation of selenium vacancies without introducing extrinsic dopants. The computational predictions are validated through solid‐state synthesis of Bi₂SeO₂. More than two orders of magnitude increase are observed in the electron concentration simply by adjusting the synthesis conditions. Additionally, a significant effect of grain boundary scattering on the electron mobility in Bi₂SeO₂is revealed, which can also be controlled by adjusting T_SSR. By simultaneously optimizing the electron concentration and mobility, azTof ≈0.2 is achieved at 773 K for self‐dopedn‐type Bi₂SeO₂. The study highlights the need for careful control of thermodynamic growth conditions and demonstrates TE performance improvement by varying synthesis parameters according to thermodynamic guidelines.