Over the past century Louisiana has lost nearly 5000 km² of coastal marsh, half of which was caused by waves impacting the marsh edge. Due to less intense freeze events the subtropical Avicennia germinans is expanding along the U.S. Atlantic and Gulf coasts with significant implications including a reduction in marsh erosion. Here we show that erosion is reduced by 40-60% when dominated by Avicennia cover. This reduction is a result of greater belowground biomass and deeper roots which leads to stronger soil strength and less erosion. We also show that a reduction in erosion only occurs under dense Avicennia cover, with erosion rates not reduced when Avicennia only cover part of the marsh. While dense Avicennia cover will not completely prevent the erosion of coastal marshes, it will slow it down. Managers using Avicennia to reduce marsh erosion should consider their density and location prior to any restoration effort.