This study aims to research the effect of cooperative learning on the academic success of 8th grade students in the subject of triangles in mathematics. The study, in which the Mixed Method was used, was carried out on a total of 84 students studying two classes of a secondary school affiliated to the Ministry of National Education in the spring semester of 2017-2018 academic year. The study includes an experiment group and a control group. During the period of the four-week application process, the lessons are introduced using the cooperative learning method in the experiment group and using the traditional learning method in the control group. “Mathematical Success Test” developed by the researchers is held for experiment and control groups as pre-test and post-test in order to collect data. Furthermore, an open-ended semi-structured interview form is used at the end of the process to collect the opinions of students on the cooperative learning method. The data were analysed using SPSS.24 statistics program. Statistical analyses such as dependent and independent t-test, arithmetic mean, standard deviation, frequency and percentages were used to analyse the data. The collected data were tested at a significance level of 0,05. As a result of the study, a significant difference was determined in favour of the experiment group in terms of the post-test success levels. At the end of the study, experiment group students stated that they found the cooperative learning method beneficial and it helped improve sharing, communication, responsibility, and feelings of belonging and confidence.