This study investigated the relationship between autonomy and English language achievement among third-grade high school students as mediated by foreign language classroom anxiety in a city in the north-west of Iran. A sample of 400 students (187 males, and 213 females) was assessed for their levels of autonomy and foreign language anxiety using the Autonomy Questionnaire and Foreign Language Classroom Anxiety Scale (FLCAS), respectively. Participants' scores on their final English exam were also used as the measurement of their English achievement. The results of Pearson correlation revealed a strong correlation between learners' autonomy and their English achievement (r = 0.406, n = 400, p < 0.01). Also, foreign language classroom anxiety was found to be significantly and negatively correlated with English achievement (r = -0.472, n = 400, p < 0.01). Hierarchical multiple regression was used to assess the ability of autonomy to predict language learning achievement, after controlling for the influence of anxiety. In sum, the results of hierarchical multiple regressions revealed that foreign language classroom anxiety significantly mediates the relationship between autonomy and English language achievement. Implications for both teachers and learners, and suggestions for further research are provided.