The role of feelings of belonging at school in adolescents' academic and psychological functioning is well established. This study aimed to validate the School Connectedness subscale from the Maryland Safe and Supportive Schools (MDS3) School Climate Survey for secondary school students in Spain. This subscale encompasses the three key dimensions traditionally associated with school belonging. The internal structure of the scale and its measurement invariance across sex and age groups were examined. Additionally, new sources of validity evidence related to psychological adjustment were analyzed. A total of 1774 students aged 14-18 years (M = 15.70; SD = 1.26; 53.7% females) participated in the study. The results confirmed the three-factor structure of the scale, consistent with the theoretical framework proposed. Furthermore, measurement invariance was supported across sex and age groups. School connectedness was positively correlated with personal well-being and self-esteem, and negatively correlated with suicidal behavior, depressive symptoms, and emotional and behavioral problems. The resulting School Connectedness Scale is a brief, easy, and reliable tool for assessing this construct in adolescents. The findings have practical implications for assessing and promoting belongingness in educational settings.