The spread of the invasive native clonal grass Elymus athericus is one of the most significant changes that have affected the plant communities of European salt marshes in the last decade. The goal of this study was to investigate the rate of this invasion and its impact on C cycling in a non-grazed salt marsh of the Mont Saint-Michel Bay (France). Aboveground net primary production (ANPP), litter quantity and quality, and decomposition rates were quantified in Elymus athericus and in the original plant communities (“control”). Since 1991, Elymus athericus has been replacing the characteristic plant communities at a rate of 4 ha yr⁻¹. ANPP was significantly higher in Elymus athericus (3011 ± 347 gDW m⁻² yr⁻¹ and 1181 ± 123 gC m⁻² yr⁻¹) than in the control (2028 ± 239 gDW m⁻² yr⁻¹ and 771 ± 76 gC m⁻² yr⁻¹) stands. Lignin content of Elymus athericus' litter was 3.5 times greater than in the litter of the control communities, explaining its accumulation under the vegetation (i.e., litter quantity was from 2 to 10 times greater under Elymus athericus than under the control vegetation). C mineralization in the sediment (expressed as rate of CO₂ per mass of sediment) was significantly lower under Elymus athericus than under the control vegetation. Our results suggest that Elymus athericus increased C trapping within the salt marsh and thus limits the potential for C exportation toward coastal waters.