This study examined the mediating effect of perfectionism on the relationship between language learning and foreign language achievement of high school EFL learners. To this end, 400 eleventh grade high school students were recruited through cluster random sampling. They were selected from eight high schools in four cities of Iran (i.e., Tehran, Ahvaz, Semnan, and Kerman). Afterwards, two questionnaires were administered to the participants. The first questionnaire was the shortened form of Gardner’s Attitude/Motivation Test Battery (AMTB) for EFL learners, and the second one was Almost Perfect Scale-Revised (APS-R) measuring the level of perfectionism among respondents. Moreover, the participants’ scores on the English final exam held by Iran’s Ministry of Education was considered as the indicator of foreign language achievement. The obtained data were analyzed through Pearson correlations and bootstrap resampling statistical method. The results indicated a positive correlation between all variables. Furthermore, it was revealed that language achievement and language learning motivation were partially mediated by perfectionism.