The few studies on the achievement differences of mainstream and immigrant primary school students in large-scale assessments point to an achievement gap in many subject areas, with immigrant students trailing mainstream students. A recurrent methodological problem of these studies is that the two student groups do not come from comparable backgrounds. This study examined the Trends in International Mathematics and Science Study (TIMSS) math achievement of 4th grade mainstream and immigrant students in various European countries. Results showed that there was no systematic difference between first- and second-generation immigrants' mathematics achievement levels. It was found that after statistically matching the background resources of the mainstream and immigrant students, both groups revealed comparable levels in mathematics. Finally, related to country characteristics that were associated with educational achievement, it was found that immigrant students in countries which had higher human development indicators showed higher educational performance.