Geophysical observations indicate that iron enrichment of various spatial scales may be present in the lowermost mantle. Various mechanisms have been proposed to explain the process of iron infiltration from the core to the mantle, though each with its own inherent limitations. Grain boundary (GB) diffusion significantly outpaces bulk diffusion within crystal interiors, and may facilitate iron transport across the core-mantle boundary (CMB). In this study, we investigate diffusion in two symmetric tilt GBs in ferropericlase using large-scale molecular dynamics simulations. The GB diffusivities of Mg and O of periclase as well as their temperature dependence agree well with previous studies. In addition, we study the GB diffusion of Fe in (Mg,Fe)O GBs for the first time. The results suggest that GB diffusion of Fe is likely to be sluggish near the CMB, and thus may not be an effective mechanism to transport iron from the core to the mantle.