This synthesis aims to see the effects of Socio-scientific Issues-Based Instruction (SSI) on the academic achievement of students in Turkey and to help shape future studies on this topic. A mixed-research synthesis method was employed in this study. As a result of the data searching process, 12 studies with quantitative data and 6 studies with qualitative were included in the synthesis. The quantitative studies were analysed through a meta-analysis and the qualitative studies were analysed through thematic synthesis method. As a result of the meta-analysis it was determined the SSI had strong and significant on (g=1.076, 95% CI [0.724, 1.428]) on academic achievement. In addition, it was determined that the calculated common effect size differed significantly according to the school level and the education program applied. As a result of the thematic synthesis six analytic themes were formed. It was attempted to explain the variance between studies included in the meta-analysis, using the analytical themes obtained from the thematic synthesis. As a result of the analysis, it was determined that SSI should be done outside the classroom, experts should be invited to these activities, teachers should be trained on this subject and a special education program should be prepared.