Background: According to the risk and resilience perspective, protective factors can attenuate the effect of risks and challenges on children's adjustment. For immigrant and refugee children, supportive relationships in the new context can be particularly beneficial. We expected that supportive school relationships play a protective role for the school adjustment of first-generation immigrant and refugee and non-immigrant children, by moderating the effect of one acculturative challenge (i.e., school language proficiency). Aims: We investigated the moderating roles of teacher-child and peer relationship quality on the association between German language proficiency and school adjustment (i.e., school achievement and belongingness) among first-generation immigrant and refugee and non-immigrant children. Sample: We recruited n = 278 fourth grade children (M[subscript age] = 10.47, SD[subscript age] = 0.55, 53.24% female; 37% first-generation immigrant and refugee children). Methods: Questionnaires assessed children's reported teacher-child and peer relationship quality and school belongingness. One vocabulary test measured children's German proficiency. School achievement was assessed by their grade point average (GPA) and by a reading comprehension test. To investigate our hypotheses, we performed path analyses. Results: Teacher-child relationship mitigated the effect of German proficiency on children's reading comprehension among all children and on GPA among immigrant and refugee children only. Peer relationship buffered the negative effect of German proficiency on school belongingness. Conclusions: Teacher-child and peer relationship quality can be beneficial for the school adjustment of both first-generation immigrant and refugee children and non-immigrant children.