Regarding the attractiveness of educational games for EFL learners, the present research intended to investigate the effects of gamification on improving Iranian EFL learners’ idiomatic knowledge. In addition, the current research examined the attitudes of Iranian EFL learners towards using gamification in English learning in the Iranian EFL context. Firstly, to carry out this research, the researchers administered the Oxford Quick Placement Test (OQPT) to 84 Iranian EFL learners and selected 50 upper-intermediate participants. Secondly, they divided the selected participants into an experimental group (EG) and a control group (CG). Thirdly, the researchers gave an idiom pretest to the participants of both groups to assess their idiomatic knowledge before starting the treatment. Fourthly, the EG was taught the new idioms using game-based instruction, while the CG received traditional idiom instruction. Following the instruction of 70 idioms to both groups, the participants received an idiom posttest to determine the influence of the treatment on their idiomatic knowledge. The EG was then provided an attitude questionnaire, and 15 members of the EG were interviewed as a result of the questionnaire. In the idiom posttest, the data were evaluated using paired samples and independent samples -tests, and the findings revealed that the EG performed significantly better than the CG on the posttest of idioms. In fact, gamification helped EFL learners develop their idiomatic knowledge. The results indicated that Iranian EFL learners presented positive attitudes toward gamification in learning English idioms. In addition, the results revealed that enjoyment, fun, reducing anxiety, involvement, and immediate feedback were the factors that the students mentioned for the instructional games. The implications of this research can encourage English teachers to implement different educational games in their teaching.