Fish are rich in the micronutrients required to alleviate nutrient deficiencies but remain untapped in nutrition security discourses due to the paucity of supporting empirical data and evidence. In this thesis, I aimed to untangle factors influencing the consumption of fish to help alleviate food insecurity and nutritional deficiencies. Firstly, I sought to establish how much fish is available in Kenya, and what contribution this can make to ensure food and nutrition security at the national level. Secondly, I examined the role of subnational fish markets in facilitating the availability and accessibility of nutritious fish within coastal populations. Thirdly, I investigated how social processes and power dynamics influence people’s access to fish, and their agency to acquire enough nutritious and culturally appropriate seafood among coastal communities. Finally, I assessed how household fish consumption and socio-demographics influence the contributions fish make to human health and identified successful coping mechanisms. I utilized interdisciplinary mixed approaches and drew from four theoretical frameworks (Sustainable Food System Framework, a Theory of Access, the Capability Approach, and Theory of Change) to reveal factors that influence whether fish can be used for food and nutrition security. The findings reveal that the fish supply in Kenya is species-diverse, varies nutritionally, and has the potential to help support food and nutrition security among vulnerable or targeted populations. I unravel how the context of the studied populations influences the availability, access, agency, and utilization of fish, to affect food and nutrition security. I identify policy levers for improved management and governance, that include bringing together the usually disparate food systems sectors spanning food science, public health, fisheries science, policy, and governance. This study provides empirical evidence to support a transformation in Kenya’s fish food system. Further research to address identified limitations is proposed.