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The primary productivity of coastal wetlands is changing dramatically in response to rising atmospheric carbon dioxide (CO₂) concentrations, nitrogen (N) enrichment, and invasions by novel species, potentially altering their ecosystem services and resilience to sea level rise. In order to determine how these interacting global change factors will affect coastal wetland productivity, we quantified growing-season carbon assimilation (≈gross primary productivity, or GPP) and carbon retained in living plant biomass (≈net primary productivity, or NPP) of North American mid-Atlantic saltmarshes invaded by Phragmites australis (common reed) under four treatment conditions: two levels of CO₂ (ambient and +300 ppm) crossed with two levels of N (0 and 25 g N added m⁻² yr⁻¹). For GPP, we combined descriptions of canopy structure and leaf-level photosynthesis in a simulation model, using empirical data from an open-top chamber field study. Under ambient CO₂ and low N loading (i.e., the Control), we determined GPP to be 1.66 ± 0.05 kg C m⁻² yr⁻¹ at a typical Phragmites stand density. Individually, elevated CO₂ and N enrichment increased GPP by 44 and 60%, respectively. Changes under N enrichment came largely from stimulation to carbon assimilation early and late in the growing season, while changes from CO₂ came from stimulation during the early and mid-growing season. In combination, elevated CO₂ and N enrichment increased GPP by 95% over the Control, yielding 3.24 ± 0.08 kg C m⁻² yr⁻¹. We used biomass data to calculate NPP, and determined that it represented 44%–60% of GPP, with global change conditions decreasing carbon retention compared to the Control. Our results indicate that Phragmites invasions in eutrophied saltmarshes are driven, in part, by extended phenology yielding 3.1× greater NPP than native marsh. Further, we can expect elevated CO₂ to amplify Phragmites productivity throughout the growing season, with potential implications including accelerated spread and greater carbon storage belowground.