The cordgrass Spartina anglica C.E. Hubbard (Poaceae) is an invasive transformer in many salt marsh ecosystems worldwide. Relatively little is known about the capacity of Spartina to accelerate salt marsh succession and to protect salt marshes against sea level rise. We analyzed long-term changes in vegetation and elevation in mainland salt marshes of the European Wadden Sea in Schleswig-Holstein, Germany, to estimate the impact of non-native Spartina on the geomorphological resistance of salt marshes to sea level rise and on changes in species diversity. From 1989 to 2019, the Spartina-zone shifted and expanded upwards to elevations of the high marsh zone and Spartina increased in frequency in several salt marsh vegetation communities. At sites where Spartina dominated the vegetation already three decades ago, elevation and species diversity increased with a higher rate compared to sites lacking Spartina. The median change rates reached for elevation MHT +8.6 versus +1.5 mm per year, for species richness +3 versus ±0 species per three decades, and for evenness +0.04 versus −0.08 per three decades, regarding plots with versus without former Spartina dominance, respectively. Invasion of salt marshes by Spartina and its continued, long-term presence were associated with increased elevation and species diversity in the face of sea level rise.