Although anti-immigrant attitudes represent a widespread social problem in many European societies, research has only partially understood the role the demographic size of the immigrant population plays for the prevalence of such attitudes. In this study, we use group threat- and intergroup contact theory to derive competing hypotheses on the role the size of the immigrant population plays for explaining the anti-immigrant attitudes of Dutch citizens. To this end, we used structural equation modeling with robust standard errors on nationally representative individual-level survey data enriched with official municipality-level statistics. We found empirical evidence for both group threat- and intergroup contact theory. Objective measurements of immigrant group size corresponded with subjective perceptions of a larger immigrant group size. Conversely, subjective perceptions of a larger immigrant group size were associated with perceptions of threatened group interests, which in turn related to anti-immigrant attitudes. On the other hand, however, larger immigrant group size facilitates intergroup contact, which was negatively associated with perceived threat and subsequent anti-immigrant attitudes.