Recent observational studies have suggested a role for soil moisture and land--atmosphere coupling in the 15-day westward-propagating mode of intraseasonal variability in the West African monsoon (WAM). This hypothesis is investigated with a set of three atmospheric general circulation model (AGCM) experiments. 1) When soil moisture is fully coupled with the atmospheric model, the 15-day mode of land--atmosphere variability is clearly identified. Precipitation anomalies lead soil moisture anomalies by 1--2 days, similar to the results from satellite observations. 2) In a sensitivity experiment, soil moisture is externally prescribed with all intraseasonal fluctuations suppressed. The 15-day precipitation signal is still present, as a purely internal atmospheric free mode, though its propagation is less coherent. 3) In a final experiment, the atmospheric model is forced with a 15-day westward-propagating cycle of regional soil moisture anomalies based on the observed mode. Through a reduced surface sensible heat flux, the imposed wet soil anomalies induce negative low-level temperature anomalies and increased pressure (a cool high). An anticyclonic circulation then develops around the region of wet soil which enhances northward moisture advection and convection to the west. This favours westward propagation, such that the atmospheric 15-day free mode becomes weakly phase locked to the imposed soil moisture forcing. Hence, although the 15-day wave can exist as a purely internal atmospheric mode, soil moisture and land--atmosphere coupling act to further organise the mode and increase its coherence.