Undernutrition and food insecurity are critical problems among under-five children in many developing countries. This study was carried out for eight months between 2009 and 2011. It combined quantitative data (N=156) and qualitative data to explore nutritional status among under-five children in Jibondo and Chole villages within Mafia Island Marine Park in Tanzania. Weight-for-age analysis was performed according to World Health Organization standards to determine the proportion of underweight children among the samples. The prevalence of underweight children was high in both villages, and in Jibondo village it was even higher (69%) than in Chole village (40%). Interviewees attributed the exceptionally high underweight problems in Jibondo to a substantial reduction in breast-feeding frequency. This was because mothers resumed seaweed farming and octopus fishing soon after delivery. Consequently, infants were fed poor-quality nutritional substitute foods at a tender age. Decreased family income, food insecurity, changes in gender roles and increased responsibilities for women were also perceived to be key underlying problems contributing to higher levels of undernutrition among children in the study areas. If healthy generations and sustainable social-ecological systems are to be achieved within the Park in the future, policies that review fishing restrictions, improve fish trading, increase household food security and liberate mother’s time for breastfeeding and child-caring activities would be essential to address the current undernutrition among the under-fives.