Coastal wetlands are hot spots of carbon sequestration, plant growth, and biogeochemical cycling due to their position at the interface of fresh and ocean waters. High vegetation productivity fueled by abundant water and nutrients combined with anoxic sediment conditions that slow decomposition drive high carbon sequestration potentials. Coastal wetlands can also be significant sources of greenhouse gas emissions, particularly methane (CH4) and nitrous oxide (N2O), which are produced by anaerobic processes. However, coastal wetlands are not currently represented in Earth System Models (ESMs) such as the Energy Exascale Earth System Model (E3SM). ESMs therefore likely overestimate riverine N inputs to the coastal ocean and underestimate C sequestration and greenhouse gas emissions (N2O and CH4) in coastal regions. To address these issues, we have implemented coastal wetland processes including redox interactions, tidal fluctuations, and vegetation responses to salinity, inundation, and sulfide concentrations into the E3SM Land Model (ELM). Our simulations integrate a chemical reaction network including aerobic decomposition, nitrification, denitrification, iron reduction, sulfate reduction, and methanogenesis. Simulations of biogeochemistry under tidal regimes compared well with field measurements of porewater concentrations in salt marshes. In model simulations, increases in saltwater influence drove enhanced sulfate availability that suppressed methane production, increased methane consumption, and increased organic matter decomposition rates. Vegetation stress responses to salt and sulfide accumulation allow the model to simulate ecosystem productivity responses to tidal inundation and flushing of coastal wetland soils. These results show the importance of representing complex biogeochemical cycles and their interactions with vegetation and its adaptations to salinity and inundation in coastal wetland systems.