The study investigated systems thinking as a predictor of security management effectiveness in tertiary institutions in Cross River State, Nigeria. The study was motivated by the increasing complexity of security challenges confronting tertiary institutions and the need for management approaches capable of enhancing institutional security outcomes. The study adopted a correlational survey research design. The population comprised 6,215 academic and non-academic staff of selected tertiary institutions in Cross River State, Nigeria. A sample of 376 respondents was selected using a multistage sampling procedure involving purposive, stratified, proportionate and simple random sampling techniques. Data were collected using a researcher-developed instrument titled Systems Thinking and Security Management Effectiveness Questionnaire (STSMEQ).The instrument was validated by experts in Educational Management, Measurement and Evaluation and Security Studies, while it's reliability was established using Cronbach's Alpha reliability coefficient of 0.86. Data were analyzed using Pearson Product Moment Correlation Coefficient and Simple Linear Regression Analysis at 0.05 level of significance. The findings revealed a significant positive relationship between systems thinking and security management effectiveness in tertiary institutions in Cross River State, Nigeria (r=.624, p < .05). The regression analysis further showed that systems thinking significantly predicted security management effectivenes (Beta = .624, t = 15.452, p < .05), accounting for 38.9% of the variance in security management effectiveness (R² = .389). The study concluded that systems thinking is a significant determinant of security management effectiveness and that institutions characterised by holistic thinking, interconnectedness awareness, feedback orientation, strategic thinking, and collaborative problem-solving are more likely to achieve effective security management outcomes. The study recommended that tertiary institutions promote systems-thinking approaches in decision-making, security planning, policy implementation, and stakeholder collaboration to enhance campus security and institutional resilience.