Structural, electronic, and thermodynamic properties of Sc 1−x Al x As alloys were studied using first‐principles approaches. The calculations are based on full‐potential linearized‐augmented plane wave (FP‐LAPW) method, within density functional theory (DFT). The exchange‐correlation effect is treated by both local‐density approximation (LDA) and generalized‐gradient approximation (GGA). In the latter approach, both Perdew‐Burke–Ernzerhof (PBE) and Engel–Vosko (EV) functionals of the exchange‐correlation energy were used. Given that the binary precursor compounds ScAs and AlAs crystallize in rock‐salt and zinc‐blende, respectively, we made calculations for the ternary alloys in these two phases. The effect of composition x on structural parameters, band‐gap energies, mixing enthalpies, and phase diagrams was analyzed for 0, 0.25, 0.5, 0.75, 1. The effect of atomic composition on lattice constant, bulk modulus, and band‐gap energy shows nonlinear dependence on composition x . Deviations of the lattice constant from Vegard's law and deviations of the bulk modulus and gap‐energy from linear concentration dependence (LCD) were found. We have found a metallic character for rock‐salt Sc 1−x Al x As alloys, while the zinc‐blende Sc 1−x Al x As alloys are semiconductors. Our results show that the band‐gap undergoes a direct ( )’to‐indirect ( ) transition at a given aluminium composition. The physical origin of the band‐gap bowing in zinc‐blende Sc 1−x Al x As alloys was investigated. To study the thermodynamic stability of Sc 1−x Al x As alloys, a regular‐solution model was used. The calculated excess mixing enthalpy is positive over the entire aluminum composition range. \end Newabstract