We examined the incremental effect of academic interest on achievement beyond general cognitive ability and students’ background characteristics in five domains (math, German, biology, chemistry, and physics). We analyzed a nationally representative German dataset of 39,192 ninth-grade students and found a unique effect of interest over and above the other predictors across the five domains, both for class grades and standardized test scores. The effect was present between persons (in a given domain, students with higher interest showed higher achievement) and within persons (the same student showed a higher achievement in domains she/he was more interested in). The effects were stronger for grades than test scores and stronger in math than in other domains. The results emphasize the positive relation between interest and academic achievement in different domains. Furthermore, they expand the literature by emphasizing the role of the achievement measure and the domain as moderators of the interest–achievement relation and by showing that interest can predict both inter- and intraindividual variation in achievement.