This study examined the relationships between student engagement in academically relevant activities and their academic achievement in the first year of university in Cambodia, and tested the presence of conditional effects of student engagement on achievement by students' gender, geographical origin, and pre-university academic experience. The participants were 919 first-year students at nine universities in Phnom Penh City. Student engagement was measured using a revised version of the National Survey of Student Engagement and the College Student Experiences Questionnaire. The regression analyses revealed that the student engagement in time spent on out-of-class course-related tasks, homework/tasks, and active participation in classroom settings added significant values to Cambodian student achievement. Contrary to the extensive student engagement literature in developed countries, student engagement in out-of-class peer learning and extensive reading did not make any meaningful impacts on student achievement in the present study. Certain effects of student engagement on achievement tended to differ in magnitude by students' pre-university academic experience and geographical origin. Implications for policy are discussed with a call for solid programs to promote and enhance the quality of student engagement activities on-and off-campus, especially among the academically disadvantaged students.