Isostatic pressure effects on the elastic and electronic properties of non-doped and Mn4+-doped K2SiF6 (KSF) have been investigated by first-principles calculations within density functional theory (DFT). Bulk modulus was obtained by the Murnaghan’s equation of states (EOS) using the relationship between volume and pressures at pressures between 0 and 40 GPa, and elastic constants were calculated by the stress–strain relationship giving small distortions at each pressure point. The other elastic parameters such as shear modulus, sound velocity and Debye temperature, which can be obtained from the elastic constants, were also estimated. The influence of external isostatic pressure on the electronic properties, such as crystal field strength 10Dq and emission energy of 2E → 4A2 transition (Eem), of KSF:Mn4+ was also studied. The results suggest that 10Dq and Eem linearly increase and decrease, respectively, with increasing pressure.