This study aimed to identify the extent student diversity and school climate vary within and across France, Italy, Spain, and Turkey, to investigate and compare the impact of student diversity on school climate in the selected Mediterranean countries. A quantitative research design was adopted by using the data of 41,789 teachers obtained from the dataset of the Teaching and Learning International Survey (TALIS) 2018. The research questions and hypotheses were formulated to investigate the extent of student diversity and school climate variations within and across the selected countries. Accordingly, the variability of general school climate, teacher-student relations, teaching experience in multicultural classrooms, teachers' self-efficacy in multicultural classrooms, the presence of students from various ethnic backgrounds, and diversity practices were tested in the schools of France, Italy, Spain, and Turkey in the research model. Statistical analyses, including chi-square and t-tests, ANOVA, and multiple regression analysis, were conducted to test the hypotheses and examine the relationships between student diversity and school climate. The findings revealed significant variations in student diversity and school climate within and across the participating countries, and evident relationships between student diversity and school climate were discovered and compared among the selected Mediterranean countries. This study contributes to the knowledge base of educational policymakers, administrators, practitioners, and teachers by guiding them in creating inclusive and supportive learning environments that promote positive student experiences and academic success and suggesting the need for policies and practices that support inclusive education and foster positive school climates in multicultural classrooms.