The promotion of students' engagement with school is an internationally acknowledged challenge in education. There is a need to examine the structure of the concept of student engagement and to discover the best practices for fostering it across societies. That is why the cross-cultural invariance testing of students' engagement measures is highly needed. This study aimed, first, to find the reduced set of theoretically valid items to represent students' affective and cognitive engagement forming the Brief-SEI (brief version of the Student Engagement Instrument; SEI). The second aim was to test the measurement invariance of the Brief-SEI across three countries (Denmark, Finland, and Portugal). A total of 4,437 seventh-grade students completed the SEI questionnaires in the three countries. The analyses revealed that of the total 33 original instrument items, 15 items indicated acceptable psychometric properties of the Brief-SEI. With these 15 items, cross-national factorial validity and invariances across genders and students with different levels of academic performance (samples from Finland and Portugal) were demonstrated. This article discusses the utility of the Brief-SEI in cross-cultural research and its applicability in different national school contexts.