Though documented extensively using self-report measures - especially in post-secondary course contexts - the relations among students' interest, engagement, and academic achievement in high school science courses has been studied far less - especially when considering online versions of these courses. Self-reported interest collected at the beginning of the semester, process measures of emotional, behavioural, and cognitive engagement measured with textual analysis using the Linguistic Inquiry and Word Count (LIWC) measures of online discussion threads and time spent on the learning management system were collected throughout the semester, and final course grades were collected at the end of the semester. A sample of 622 high school students were recruited from five STEM courses to participate in this study. Results indicate that interest predicts behavioural engagement, behavioural and cognitive engagement predict final course grade, and interest predicts final course grade. Findings extend research by describing the role of engagement in online high school STEM courses.