Summary The cubic Sr2XNbO6 (X = La, Lu) double perovskite oxides (DPOs) have been examined by density functional theory (DFT). Structural, elastic, electronic, thermoelectric (TE), and optical characteristics are computed by utilizing the full‐potential linearized augmented plane wave method (FP‐LAPW). It is probed that Sr2LaNbO6 and Sr2LuNbO6 are semiconductors with direct band gap (Eg) of 4.02 and 3.7 eV, respectively at theΓ symmetry points. To ensure the structure's stability, the Goldschmidt tolerance factor (τ) and the enthalpy of formation energy (∆E) are determined. Shear modulus (G), poisson's ratio (χ), elastic coefficient (Cij), anisotropy (A), bulk moduli (B), young's modulus (Y), and pughratio (B/G) are computed. Furthermore, thermoelectric (TE) features such as seebeck coefficient (S), figure of merit (ZT), electrical conductivity (σ), power factor (PF), and thermal conductivity (K) are determined by using BoltzTrap code. The estimated values of ZT reveal that Sr2LuNbO6 is a more promising material for TE applications. Optical characteristics such as absorption coefficient α(ω), dielectric constant ε(ω), optical conductivity σ(ω), refractive index n(ω), and reflectivity R(ω) are also calculated. Computation indicated that Sr2XNbO6 (X = La, Lu) are promising materials for TE and optoelectronics applications in the UV region.