Cognitive appraisal has been shown to mediate the relationship between stressors and internalizing symptoms, but not among newcomer immigrant youth facing ambiguous discrimination. Using a mixed-methods design with a sample of newcomer adolescents from African, Arab, Asian, Caribbean, European, and Latin American countries, this study measured the perceived occurrence of discrimination (attribution to discrimination) and its perceived degree of severity (cognitive appraisal) in relation to internalizing symptoms and academic achievement. Attribution to discrimination predicted greater internalizing symptoms, and appraisal mediated this relationship. The mediation effect was found for younger, but not older adolescents. Quantitative results suggest that perceiving discrimination to be threatening, not just that it occurred, relates to psychological health. Qualitative results suggest that for newcomer adolescents, discrimination is a highly prevalent and complex stressor with multiple psychosocial ramifications.