This study investigated the effect of self-efficacy levels on academic achievement of senior secondary school students exposed to Learning together strategy. The population consisted of all SSII Civic Education students in public coeducational secondary schools in Lagos State's senatorial districts. Two schools were purposively selected within the area of study. The research design was a quasi-experimental design using the pre-test, post-test non-equivalent group design. The instrument used to gather data was the Civic Education achievement test (CEAT) and General Self Efficacy Scale (GSES). A total of 91 SS2 civic education students drawn from intact classes participated in the study. The study found a statistically significant effect of self-efficacy levels on the academic achievement of students exposed to Learning together Strategy. Hence, the study concluded that self-efficacy plays a crucial role in determining students' academic success when using the Learning Together strategy. Based on the findings, it was among others that Civic education teachers should focus on fostering and enhancing students' self-efficacy beliefs through offering opportunities for collaboration in the classroom