This paper reports on a novel method for determining the specific surface area (SSA) of shale via headspace gas chromatography (HS-GC). The method is based on the water adsorption on the surface of shale sample after achieving phase equilibrium at an elevated temperature (i.e., heating at 125 °C for 48 h). A mathematical model shows that the SSA can be determined from the signal of the vapor water released during HS-GC analysis. The results obtained by this method correlated well (R2 = 0.992) with data obtained by the reference BET method. Because the phase equilibrium step for multiple samples can be conducted simultaneously, and because the phase re-equilibrium step is much faster in the HS-GC measurement, the present method is more efficient for batch sample testing.