Red-breasted Mergansers (Mergus serrator) commonly breed in estuaries, but little is known about their brood-rearing in coastal environments. We measured daily movements and habitat use of radio-marked (n = 17) female Red-breasted Mergansers with broods originating from coastal barrier islands at Kouchibouguac National Park, New Brunswick, Canada from 2002 to 2004. Primary brood movements from nest sites to initial rearing areas were often extensive, averaging 3.5 km (n = 15), since many broods crossed Saint-Louis Lagoon to continental rearing sites. Broods remained mobile throughout the rearing period and there was little difference in daily movements between age class I (days 1–10 post nest exodus), class II (days 11–20), and class III (>20 days) broods. Broods frequented shallow (x̄ = 51 cm, 95% CI: 44–58 cm, n = 191 locations), nearshore (x̄ = 47 m, 95% CI: 33–60 m, n = 157 locations) waters that often supported submergent eel grass (Zostera marina). Broods selected estuarine intertidal regions in Saint-Louis and Kouchibouguac lagoons, as well as wetlands at the mouths of tidal streams. Few broods were found in tidal river and marine habitats. Continental estuarine intertidal, tidal stream, and saltmarsh habitats were preferred by age class I broods whereas estuarine intertidal and subtidal habitats were preferred by age classes II and III. This study highlights the importance of estuarine habitats in lagoons and tidal streams for brood-rearing Red-breasted Mergansers in eastern New Brunswick.