Mg2Si is an important semiconducting silicide material with promising applications in optoelectronics, thermoelectrics, and photovoltaics. Mg2Si(001)-(1 × 1) surfaces have been suggested to be metallic from density functional (DFT) studies performed within local density (LDA) and generalized gradient approximations (GGA). Here in this article, we revisit and further explore surface electronic structure, surface reconstruction and stability of Mg2Si(100) surfaces within DFT framework in combination with Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional as well as Tran-Blaha (TB09) meta-GGA functional for exchange-correlation. Our results suggest that reconstructed Mg2Si(100) surfaces are semiconducting which are incorrectly found as metallic in absence of reconstructions with higher surface periodicities and when computed within LDA (GGA) approximations. In particular, the Si-terminated Mg2Si(100) surface exhibits (2 × 1) reconstruction qualitatively similar to buckled asymmetric dimer type reconstruction of Si(100)-(2 × 1) surface. The band gap of Mg-terminated relaxed Mg2Si(100)-(1 × 1) and reconstructed Si-terminated Mg2Si(100)-(2 × 1) surfaces are found to be in the range ~ 0.3–0.5 eV. The Si-terminated reconstructed (2 × 1) surface is found to be lower in energy by ~0.21 J/m2 than the (1 × 1) relaxed surface. Further, localized surface states are formed in the band gap near valence band maximum (VBM) as well as at ~7 eV below the VBM depending on the surface termination. The surface localized gap states may be expected to have important implications for relaxations and thermodynamic energies of Mg2Si(100) surfaces. © 2019 Elsevier Masson SAS