Math competence beliefs and achievement are important outcomes of school‐based learning. Previous studies yielded inconsistent results on whether skill development, self‐enhancement, or reciprocal effects account for the interplay among them. A development‐related change in the direction of their relation in the early school years might explain the inconsistency. To test this, 542 German elementary school students (M = 7.95 years, SD = 0.58) were repeatedly investigated over 24 months from Grade 2 to Grade 4. Math competence beliefs declined and had a growing influence on subsequent math grades. This suggests changes in the dominant direction of the relation from a skill development to a reciprocal effects model during elementary school. Findings are discussed with regard to their theoretical and practical implications.