Artificial structures, such as seawalls, are increasingly disrupting the transition zones between terrestrial and marine systems. They can impede the transport of resources across habitat boundaries and impact adjacent sedimentary ecosystems by modifying hydrodynamics which, in turn, influence sedimentology and erosion. We assessed how structural elements of Avicennia marina mangrove forests along the Parramatta River estuary, Sydney, Australia, differ in the presence or absence of a seawall on the landward side of the forest. These forests are of importance to resident and transient fauna. Sampling of paired mangrove forests, with and without seawalls, supported our hypotheses of structural differences between them. Mangrove forests with seawalls were in some instances less than a third of the width of unconstrained mangrove forests, and had up to twice the pneumatophore density. They often contained less leaf litter and had fewer saplings than forests without seawalls. These results suggest that as shoreline armouring continues, urban mangrove forests and their important ecosystem functions may be negatively impacted. Studies are now needed to ascertain the mechanisms by which seawalls modify these systems.