Open marsh water management (OMWM), a method of mosquito reduction through habitat alteration, is widely practiced in Mid-Atlantic salt marshes. The effect of these habitat modifications on obligate salt marsh breeding birds is poorly understood. The present study was conducted to quantify the extent of OMWM and determine if the technique affected Seaside Sparrow (Ammodramus maritimus) reproductive success and nesting ecology. Seaside Sparrow territory density, nest density, nest survival, productivity and vegetation cover was estimated on 19 plots (1–3 ha) within tidal marshes in Sussex County, Delaware in May–August 2006–2007. Plots were characterized as limited (N = 10) or extensive OMWM (N = 9) based on the amount of alterations and time since manipulations. Seaside Sparrow territory density and nest density were two times greater on limited OMWM plots than extensive OMWM plots. Also, the number of Seaside Sparrow eggs/ha and fledglings/ha were greater on limited OMWM plots. Seaside Sparrow nest survival rates did not differ between limited and extensive OMWM plots, but reproductive output (# nests/ha, eggs/ha, and fledglings/ha) was greater on areas with limited OMWM. OMWM may be an alternative means of mosquito control but should not be considered a method of habitat enhancement for Seaside Sparrows and possibly other obligate salt marsh bird species.