This study investigated International Baccalaureate Diploma Program students' perceptions of international-mindedness. The research used quantitative methods to explain perceptions of international-mindedness within three participating schools (two national schools and one international school) in Istanbul, Turkey. Using the conceptual framework of international-mindedness developed by Singh and Qi, the study applied a pre and post-test design to measure intercultural understanding and global engagement with Intercultural Development Inventory and Global-Citizenship Scale, respectively. The study compared data from schools with a Turkish (national) student body to a school that had international students and found no significant difference between and among students' pre and post levels of intercultural understanding and global engagement in terms of improvement after one year of International Baccalaureate Diploma Program education. The study provides important implications for practice to the other researchers and educators about the conceptualization, implementation, and assessment of international-mindedness for promoting global-citizenship pedagogy in different school contexts.