In this article, the effect of horizontal dispersion in an aquitard (HDA) on reactive solute transport in an aquifer-aquitard system is investigated. The governing equations in both the main aquifer and the aquitard incorporate terms accounting for advection, horizontal and vertical dispersions, first-order irreversible decay and linear sorption. The analytical approach is developed by applying the Laplace transform along with cosine Fourier transforms. The Laplace domain solutions are inverted to the real-time domain solutions using the Stehfest algorithm. Generally, the results reveal that the role of HDA is generally not significant on solute transport. Nevertheless, its effect depends on the aquitard/aquifer thickness ratio, the ratio of the aquitard horizontal dispersion to the aquifer longitudinal dispersion, as well as Peclet numbers of both the aquifer and the aquitard. At small Peclet numbers of the aquifer and/or large ratios of the aquitard horizontal dispersion to the aquifer longitudinal dispersion and/or large Peclet numbers of the aquitard, ignoring HDA may cause some moderate errors in concentration and stored mass within the aquitard, especially at longer times. As a consequence, the error varies across the aquitard width and depends on changes in slope of the vertical concentration profile there.