Investigations of gas transport and fate processes in packed soil systems require knowledge of the gas diffusion coefficient, D_P, as a function of air‐filled porosity, ϵ. On the basis of the literature, data from six studies over the porosity range of 0.1 to nearly 1.0, it is reconfirmed that the Marshall (1959) model better predicts D_P(ϵ) in completely dry, repacked porous media than do the Penman (1940) and Millington (1959) models. The smaller D_P value in wet soil, as compared with dry soil at the same air‐filled porosity, is accounted for by introducing a water‐induced linear reduction (WLR) term, equal to the ratio of air‐filled porosity to total porosity, in the D_P(ϵ) model. By adding the WLR term in each of the three D_P(ϵ) models for dry porous media, the so‐called WLR(Marshall), WLR(Penman), and WLR(Millington) D_P(ϵ) models for wet soil are developed. To test the three WLR models, D_P was measured at different soil‐water contents in six differently textured (6–38% clay) repacked soils. The WLR (Marshall) model accurately and best described D_P(ϵ) for all six soils and additional soils from the literature. All three WLR models performed better than previous D_P(ϵ) models. This study implies that the smaller D_P in a wet soil, which is due to water‐induced changes in air‐filled pore shape and pore connectivity, can be described by a simple, linear function of relative air‐filled porosity. The WLR(Marshall) model represents a conceptual and accurate model to predict D_P(ϵ) in sieved, repacked soil.