Coastal marsh extent is a key variable in the protection of inland areas from storm surge, flooding, and erosion. Vegetation composition is also an important factor in the functional characteristics of coastal marsh areas in terms of both wildlife value and wave energy attenuation. As pressures from accelerating sea level rise and anthropogenic stressors grow, the protection of coastal wetlands will be increasingly essential in mitigating coastal change and preserving habitat to support fisheries and other wildlife. The increase in extent of the aggressive Phragmites australis (Cav.) Trin. ex Steud. (Common Reed) in several of Mississippi’s coastal marsh areas raises questions about how these marshes may evolve in the future, in terms of both spatial extent and ecosystem function. The goals of this study were to quantify the change in extent of P. australis along the West Pascagoula River using aerial image data and to identify the areas of greatest expansion. To assess the possible influences of salinity and wave energy, two areas were examined: one at the mouth of the river delta as it enters the Mississippi Sound, and one more protected inland area with lower salinity. A supervised maximum likelihood algorithm was used to create land cover classifications at 1 meter GSD for calendar years 1996, 2010, 2014 and 2018, with land cover classified as P. australis, other marsh vegetation, marsh-upland ecotone, wooded areas, or water. Per-pixel change detections were computed among the classification images to assess areal change in P. australis extent as compared with other land cover types. Results show an increase in P. australis extent of 157 ha (approximately 1,960%) between 1996 (8 ha) and 2018 (165 ha), with greater expansion in the inland part of the study area. These land cover classifications will also be used to quantify shoreline movement for P. australis-dominated shorelines as compared with other marsh habitats, providing insights into past and future responses of P. australis to changes in sea level compared with neighboring marsh vegetation.