Although peer assessment is widely implemented in higher education, not all students are highly engaged in it. To enhance student engagement in peer assessment, we designed and developed a web-based tool, autonomy-supportive peer assessment (ASPA), to support students' need for autonomy when they conducted peer assessment. Students' sense of autonomy, and their behavioral, emotional, and cognitive engagement in peer assessment were examined via interviews and surveys. We also examined students' academic performances, including their improvement from initial to revised essays and the quality of feedback they provided. Survey results indicated that the ASPA group (1) experienced a slightly higher sense of autonomy than the non-ASPA group, and (2) spent much more time on each evaluation criterion than the non-ASPA group. Interviews suggested that both the ASPA and non-ASPA groups were engaged in peer assessment. However, there were no statistically significant differences between the two groups in behavioral, emotional, cognitive engagement, and academic performances.