Problem based learning (PBL) has great potential to enhance learning outcomes and this potential has been investigated and proved in different disciplines by many meta-analysis studies. However, there are not any meta-analysis studies aiming to investigate the effectiveness of problem based learning in English as a Foreign Language (EFL) classrooms which is an important gap that needs to be filled. Therefore, this meta-analysis study aimed to investigate the overall effect size of PBL on achievement in EFL classrooms and to examine the possible moderator variables that might be effective on this overall effect size. Along with this aim, the studies investigating the effect of PBL on academic achievement in EFL classrooms are included and analyses were carried out with 41 data (extracted from 36 unique studies). Investigation of publication bias using various methods showed that there is no publication bias. This study showed that the effect size of PBL is 1.067 indicating a large effect which means that PBL is highly effective to enhance the language achievement of students in EFL classrooms. Moderator analyses showed that language skill is a real moderator on the effect size of PBL on EFL success while the study group, treatment duration, location of study, learning environment, and document type are not real moderators.