Despite considerable efforts to restore coastal wetlands, the ecological mechanisms contributing to the success or failure of restoration are rarely assessed. Accumulation of hydrogen sulfide in sediments may accelerate rates of marsh loss in eutrophic estuaries and is likely driven by complex feedbacks between wetland plant growth and microbial redox reactions. We used a chronosequence of restored marshes in urbanized and eutrophic Jamaica Bay (New York City, USA) to assess how sediment redox conditions change among seasons and over the lifetime of restored marshes. We also compared a stable extant marsh to one that has deteriorated over the past 50 years. We collected seasonal sediment cores from each marsh, and used a motorized microprofiling system to measure the vertical distribution of oxygen and sulfide. We fit a logistic function to each profile to estimate (1) maximum concentrations, (2) rates of increase/decline, and (3) depths of maximum increase/decline. We quantified sediment density, porosity, organic content, and belowground plant biomass, and estimated differences in daily tidal inundation among sites using water-level loggers. We found that minimum oxygen and maximum sulfide concentrations occur during summer. Sulfide concentrations were highest in sites that experienced the longest daily tidal inundation, including the degraded extant marsh and the oldest restored marsh. Spatial patterns in oxygen and sulfide were related to belowground plant biomass, supporting our hypothesis that root growth increases sediment oxygen and partially alleviates sulfide stress. Our data support the growing body of evidence that belowground plant growth may enhance the resilience of marshes to sea-level rise by increasing marsh elevation and facilitating oxygen diffusion into marsh sediments.