This study assesses the impact of the Aquaculture Business Development Programme (ABDP) on nutrition and livelihoods in 15 counties across Central and Western Kenya. Through a gender-responsive and nutrition-sensitive approach, ABDP interventions significantly improved fish production, income, and dietary outcomes. Participation was predominantly male (67.5%), with older age groups more engaged in aquaculture. Treated counties recorded significantly higher household food expenditure (p < 0.005) and dam aquaculture yields (p = 0.003), with a moderate correlation between production and income. Notably, households in treated areas achieved higher dietary diversity (p < 0.005), with women meeting Minimum Dietary Diversity for Women (MDD-W) increasing from 63% to 76.5% (p = 0.004). Micronutrient intake improved, particularly for Vitamin A-rich foods. Schools supported through ABDP showed higher fish productivity and increased access among pupils (p = 0.002). Community-led nutrition initiatives, such as kitchen gardens and cooking demonstrations, complemented these gains. A strong positive correlation (r = 0.612, p = 0.002) was observed between fish consumption and the number of Social and Behavior Change Communication (SBCC) Fish Faires conducted. Overall, ABDP demonstrated measurable success in leveraging aquaculture for rural food security, income generation, and improved community nutrition.