Academic achievement of most of the Saudi EFL learners is generally poor. A lot of research has been done to probe problems of EFL learners but very little attention has been paid to overcome these problems via better classroom environment and teaching strategies. This quasi-experimental study aimed at investigating the impact of cooperative learning on academic achievement of EFL tertiary learners at a mainstream public sector university in a Middle Eastern country. The sample of the study included 50 EFL non-English major male students enrolled at the preparatory year program in the first semester of 1434-35 corresponding to 2014 A.D. Pretest posttest experimental group research design was used for the study. Scores of the pretest and posttest for the two course-based assessments were analyzed using MS Excel 2013 and SPSS Version 20. The results of the study revealed that the experimental group showed better performance in the posttest compared to that of control group, showing that cooperative learning has positive impact on academic achievement of Saudi EFL tertiary level learners. The low achievers and the medium achievers in the EG showed statistically significant improvement after the CL treatment whereas high achievers performed equally well in both CL and traditional setting. Low achievement of Saudi EFL tertiary level learners should be given a serious consideration and proper remedial measures should be implemented. CL can be instrumental in this regard.