Abstract <p>A novel mixed hydrogen selenite–selenite cobalt hydrate, NaCo₂(HSeO₃)(SeO₃)₂·2H₂O was synthesized for the first time via a hydrothermal route and comprehensively characterized by single-crystal and powder X-ray diffraction, UV-Vis spectroscopy, and simultaneous TG–DTG–DSC analysis with evolved gas detection. Single-crystal X-ray diffraction reveals an orthorhombic structure in the <i>Cmcm</i> space group, featuring two crystallographically independent cobalt sites in octahedral oxygen coordination. The structure contains both hydrogen selenite (HSeO₃^–) and selenite (SeO₃^2–) groups, which adopt trigonal-pyramidal geometry, while water molecules occupy structural cavities and form an extensive hydrogen-bond network further stabilized by the presence of acidic hydrogen atoms. Powder diffraction confirms the phase purity of the bulk material. The optical spectrum is dominated by intense charge-transfer bands in the deep UV region, accompanied by weaker <i>d</i>–<i>d</i> transitions characteristic of high-spin Co²⁺ in an octahedral crystal field. No spectral features indicative of Co³⁺ or charge-separated Co²⁺/Co³⁺ states are observed, which is consistent with the structural and bond-valence sum analyses. Thermal analysis shows a four-step decomposition pathway involving dehydration, initial deprotonation of hydrogen selenite groups, onset of selenite destabilization with SeO₂ evolution, oxidative restructuring of the anionic framework, and final decomposition to a stable oxide residue. Compared with the nickel analogue, the cobalt compound decomposes in fewer but more energetically intense stages, reflecting the higher redox flexibility of cobalt and enhanced stability of Co–O networks. The combined structural, spectroscopic, and thermal data provide insight into the crystal chemistry of mixed hydrogen selenite–selenite cobalt systems and demonstrate that the new phase remains dominated by Co²⁺ centers throughout the studied conditions.</p>