This study sought to elucidate the multi-level factors that influence behaviors underlying high childhood stunting and widespread micronutrient deficiencies in Kiribati. This two-phase formative research study had an emergent and iterative design using the socio-ecological model as the guiding theoretical framework. Phase 1 was exploratory while phase 2 was confirmatory. In phase 1, in-depth interviews, free lists, seasonal food availability calendar workshops, and household observations were conducted. In phase 2, focus group discussions, pile sorts, participatory workshops, and repeat observations of the same households were completed. Textual data were analyzed using NVivo software; ethnographic data were analyzed with Anthropac software for cultural domain analysis. We found a combination of interrelated structural, community, interpersonal, and individual-level factors contributing to the early child nutrition situation in Kiribati. Despite widespread knowledge of nutritious young child foods among community members, households make dietary decisions based not only on food availability and access, but also longstanding traditions and social norms. Diarrheal disease is the most salient young child illness, attributable to unsanitary environments and sub-optimal water, sanitation, and hygiene behaviors. This research underscores the importance of a multi-pronged approach to most effectively address the interrelated policy, community, interpersonal, and individual-level determinants of infant and young child nutrition in Kiribati.