The relative permittivity, epsilon(r)(T, p), of liquid octane has been determined with an estimated expanded relative uncertainty of +/- 0.018 % from measurements of the complex capacitance of a concentric cylinder capacitor at temperatures between (293 and 393) K and pressures below 25 MPa. The measurements were corrected for the capacitor's isothermal compressibility that was determined by comparison of measurements of epsilon(r)(He, 303 K and 393 K, p) with ab initio results from quantum mechanics. The measured epsilon(r)(C8H18, T, p) were combined with the amount-of-substance density, rho, which was obtained from the equation of state reported by Span and Wagner (Int. J. Thermophys. 2003, 24, 41-109), to calculate the molar polarizability, P, that was fit with an expanded (k = 2) relative uncertainty of +/- 0.01 % to a correlation with three parameters. The epsilon(r)(T, p) obtained from the literature were converted to P.rho and compared with our correlation and found to differ by < +/- 0.2 % at temperatures that overlap ours.