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Summary: The electronic, optical, magnetic, and thermoelectrical properties of both, the monoclinic (α‐form) and the orthorhombic (β‐form) phases of sodium manganese (II) arsenate NaMnAsO4 cluster are investigated comprehensively by the first‐principle calculations based on the Density Functional Theory. The full‐potential linearized augmented plane waves method within the generalized gradient approximation (GGA) and GGA + U approaches have been performed by the Wien2k software. Thermodynamic stability is confirmed by means of formation energy (ΔHf). The spin‐polarization is included in the calculations to study the electronic and magnetic properties. The indirect bandgaps are observed for the monoclinic phase, then the direct bandgaps are reported at Γ‐symmetry point for the orthorhombic phase. The density of state, besides the band structure calculation, revealed that the studied material can be identified as a ferromagnetic semiconductor at low temperature with an optical bandgap energy of 1.35 and 0.83 eV for monoclinic and orthorhombic phases with GGA approach, as well as 1.27 and 1.59 eV with GGA + U approach, respectively. The refractive index range is 1.5 to 2.4 along with a wide absorption band in the UV region, which makes this material outstanding for optoelectronic devices. The thermoelectric properties for both structures have also been investigated using the BoltzTraP package. The obtained electrical conductivity, as well as the figure of merit, are found to be important for thermoelectrical devices. This suggests that NaMnAsO4 could pave the way to further experimental research on optoelectronic‐thermoelectric applications. [ABSTRACT FROM AUTHOR]