South Western Nigeria's River Basins face escalating climate risks, but adaptation by mandated River Basin Development Authorities (RBDAs) is failing. The methodological approach quantitatively assesses the institutional barriers responsible for this gap. The two objectives were to identify and quantify the severity of financial, coordination, policy and legal barriers in the Ogun-Osun and Benin-Owena basins, and to examine the statistical relationship between these barriers and the implementation of adaptation strategies. A quantitative, cross-sectional survey design was used, sampling 125 professionals from RBDAs and State Ministries. Data were collected via a validated structured questionnaire, quantified with mean variance, standard deviation and linear regression analysis. Findings revealed that Financial Barriers (Mean=4.55, SD=0.06) are the most severe, followed by Coordination Barriers (Mean=4.20, SD=0.75) and Policy and Legal Barriers (Mean= 3.45, SD=0.98 and Linear regression confirmed a significant negative relationship (R² = .456, p < .001) between barriers and implementation, within barriers. The study recommended that adaptation failure is a governance failure, primarily driven by financial and coordination-related institutional barriers and a dedicated National Water Adaptation Fund and mandatory Federal-State Basin Coordination Platforms.