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We investigate the structure and energetics of point defects in layered NaMnO₂. Our study is aimed at elucidating potential degradation mechanisms when the material is used as a positive electrode in Na-ion rechargeable batteries. Hydrogen has been implicated in this process. Using hybrid density functional theory, we find that hydrogen interstitials and Na vacancies have low formation energies and moderate migration barriers, and the proton/Na⁺ exchange process can proceed with fast kinetics. Degradation occurs when Mn moves to an antisite position, where it migrates with a low barrier of 0.18 eV. In the absence of hydrogen, formation of Mn_Na antisites is suppressed due to the high energy of the V_Mn it leaves behind, and due to the large interlayer-migration barrier for Mn. But when hydrogen is present, V_Mn is stabilized by forming V_Mn-3H complexes; in addition, the interlayer-migration barrier for Mn is substantially reduced from 1.48 eV to as low as 0.20 eV in the presence of hydrogen interstitials. Therefore, with the assistance of hydrogen, the formation of Mn_Na antisites becomes thermodynamically favorable and kinetically allowed, which eventually leads to loss of Mn²⁺.