This study examined the mediating role of student engagement in the relationship between students' perception of the instructional environment and English course achievement. The study sample consisted of 456 volunteer students studying in the 6th grade of secondary school in Türkiye. Structural equation modeling was used to examine the mediation relationship between variables. According to the analysis, REACT explains 66% of the SES variance, and both REACT and SES explain 37% of the achievement variance. Moreover, mediational pathways going through SES significantly predicted achievement. In addition, the study determined that SES played a partial mediating role in the relationship between REACT and English achievement. Based on the finding in this study that 90% of the total effect of REACT on achievement is realized through SES, it can be said that teachers can increase students' achievements in English by supporting their participation in lessons.