IntroductionDue to the COVID-19 pandemic, measures were taken that had a considerable impact on the situation in schools. In Germany, these measures lasted more than a year and ranged from school closures and distance learning to alternating teaching phases with small groups. In the present study, we examined whether third-grade students’ mathematics performance changed in different content domains before and after the COVID-19-related changes in school.MethodsIn a repeated cross-sectional design, we compared two cohorts of third graders (2019: N = 1,905; 2021: N = 3,203) based on standardized mathematics tests, constructed according to the German National Educational Standards, which allowed for a differential competence diagnostic for five content domains. Generalized linear mixed models were used to model item and person properties and assess their effects on performance.ResultsThere was a significant drop in performance overall. While the drop in the content domain Numbers & Operations was smaller than the overall drop in performance, the content domains Space & Shape and Data, Frequency, Probability were more affected.DiscussionThe findings of this study may be explained by the results of numerous surveys of students, teachers, and parents, which indicated that a lot of time was spent on exercises and reproduction tasks during distance learning.