Gas diffusion in nanoporous media is significantly different from its bulk counterpart owing to the nanoconfinement effect. In this study, a local effective diffusivity lattice Boltzmann model (LED-LBM) is proposed to explore the gas diffusion behavior under nanoconfinement. The nanoconfinement effect is incorporated into the local effective diffusivity via a spatial position-dependent mean free path. The proposed LED-LBM is validated against molecular dynamics simulation results for methane gas diffusion in a nanoscale channel. Using this model, the spatial variations in the local effective diffusivity and gas mass flux in 3D nanoporous media are visualized, and the predominant factors influencing their variation and gas diffusivity are disclosed. The results show that the spatial variation features of the local effective diffusivity and gas mass flux are strongly influenced by the pore shape and average Knudsen number. The gas diffusivity in nanoporous media is significantly lower than its bulk counterpart owing to the nanoconfinement effect, and their differences increase with the average Knudsen number. To quantify the magnitude of the nanoconfinement effect, a new concept, namely, the confinement scope, is further defined. Finally, a semiempirical formula is established to predict the gas diffusivity in nanoconfined porous media, which is expressed as a function of the bulk diffusivity, average Knudsen number, and porosity.