Studying abroad can positively influence students' personality development, transversal skills, and labour market outcomes. At the same time, students from a high social origin are more likely to study abroad than students from a low social origin. Against this background, recent research has suggested that international student mobility (ISM) may foster the reproduction of social inequality. However, this assumption has hardly been tested empirically. Drawing on social stratification theory, we first demonstrate that a scenario in which ISM increases social inequality (cumulative advantage) is as plausible as a scenario in which it decreases social inequality (compensatory levelling). We then address the sketched research gap by testing whether the effect of studying abroad on graduates' labour income varies across social groups in the German labour market. Using data from the 2005 DZHW Graduate Panel, we perform a propensity score matching and calculate random effects growth curve models to examine the role of ISM for the development of social inequality during the first 10 years of graduates' careers. In line with the scenario of cumulative advantage, our results suggest that graduates from a high social origin benefit more from ISM than graduates from a low social origin. Considering that students from a high social origin are also more likely to study abroad in the first place, we conclude that ISM tends to foster the reproduction of social inequality in the German labour market.