There are a number of individual and affective factors which correlate foreign language learners' achievement both positively and negatively. The aim of this study was to examine the relationship between perfectionism and English language achievement among high school third graders in Chenaran, a city in northeast of Iran, mediated by foreign language classroom anxiety. A sample of 239 students (110 males and 129 females) was assessed for their levels of perfectionism and foreign language anxiety using the Almost Perfect Scale-Revised and Foreign Language Classroom Anxiety Scale, respectively. Participants' scores on their final English test were also used as the measurement of their English achievement. The results did not reveal strong correlations between perfectionism and participants' English achievement (F = 0.515, p > 0.05); however, the mean English score of the participants was lower for maladaptive perfectionists. Also, foreign language classroom anxiety was found to be significantly and negatively correlated with English achievement (r = -0.357, p < 0.01). The results of analysis of variance revealed a significant positive association between perfectionism and foreign language anxiety, where maladaptive perfectionists were found to be more anxious than adaptive and non-perfectionists. Results did not support the hypothesis about foreign language anxiety as a mediator of the relationship between perfectionism and English achievement. Implications for both teachers and learners, and suggestions for further research are provided.