Gas diffusion is a dominant transport mechanism for climate and regulated gases in the vadose zone. Soil-gas diffusion is governed by the gas diffusion coefficient (Dp, m2 s−1) which is highly dependent on soil texture, such as sand, silt, clay, and organic matter contents, as well as soil physical properties such as soil-air content (ε, m3 m−3) or total porosity (Φ, m3 m−3). Soil organic matter is a key contributor to the formation of the soil pore structure (i.e., total porosity and air-filled pore tortuosity), and it highly affects Dp behavior under variably saturated conditions. In this study, based on numerous Dp data sets across soil types including sands, loamy clay soils, volcanic ash soils, and organic soils, predictive Dp models incorporating a percolation threshold (εth, m3 m−3) and pore tortuosity factor (X′) are proposed. The observed relations between either εth or X′ and either Φ or volumetric organic matter fraction (OMF, m3 m−3) were embedded in the proposed Dp model. The proposed Dp models, coupled with predictive εth and OMF models, performed well against the measured Dp data across soil types. Finally, a sensitivity analysis of the OMF in relation to the Dp and pore-network tortuosity (T) showed a reduction in Dp and increase in T with increasing OMF under the same ε conditions.