The electric‐field‐dependent longitudinal diffusion coefficient for charge carriers in a two‐valley semiconductor is calculated by modeling the carrier movement as two simultaneous coupled random walks. The first random walk represents carrier scattering from one valley to another, and the second describes the motion of a carrier within a single valley, with the carrier position in the first walk determining the parameters of the second walk. The calculated intervalley transfer diffusion coefficientDtrcontains earlier calculations ofDtras special cases. An agreement between calculated and measured low‐field diffusion coefficients for electrons inn‐GaAs at room temperature is obtained when the random time interval that a carrier spends in a valley is assumed to have a gamma distribution.