Here, we provide a systematic assessment of biaxial strain effects on the electronic, phonon, and optical properties of monolayer transition metal dichalcogenides (TMDs) XTe₂ (X = Mo and W) using density functional theory calculations. We observed a large direct bandgap of 1.163 eV and 0.974 eV for MoTe₂ and WTe_2, which reduced to 1.042 eV and 0.824 eV in the spin–orbit coupling ambient. The XTe₂ structures show a tunable bandgap with the variation of the applied biaxial strains. Due to the breaking of inversion symmetry, a large spin-valley coupling emerged at the valance band edges for both MoTe₂ and WTe₂ monolayers under applied biaxial strain. The phonon properties with different biaxial strains reveal that monolayer MoTe₂ is more stable than the WTe₂ structure. The calculated optical properties demonstrate that the dielectric constant and absorption coefficient of MoTe₂ and WTe₂ move to higher photon frequencies when the compressive strain is increased. On the other hand, with the increase in tensile strain, a red-shift behavior is found in the calculated optical properties, indicating the suitability of the XTe₂ monolayer for different infrared and visible light optical applications.