Molothrus ater (Brown-headed Cowbird) is a known nest parasite for numerous North American passerines and has exacerbated the decline of many imperiled landbird species. However, Brown-headed Cowbird habitat preferences do not frequently overlap with many salt marsh-dwelling species. During intensive demographic research of Ammodramus maritimus (Seaside Sparrow) in the coastal salt marshes of Mississippi (2010–2012), we documented the first instances of confirmed nest parasitism of Seaside Sparrows by Brown-headed Cowbirds. We suggest that sea-level rise could increase instances of nest parasitism in marsh-dwelling passerines by increasing the perimeter-to-area ratio of marsh habitat and moving existing marsh in closer proximity to habitats preferred by Brownheaded Cowbirds.