A positive academic self-concept (ASC) relates to many desirable educational outcomes. Research on which student characteristics relate to the formation of ASC is therefore crucial. To examine the importance of personality for ASC, we investigated the relation between Big Five traits and mean level as well as within-person variability in state general-school ASC for the first time using intensive longitudinal data. The sample comprised N = 291 German ninth and 10th graders who completed a 3-week e-diary after filling in a 60-item Big Five questionnaire assessing extraversion, agreeableness, conscientiousness, negative emotionality, and open-mindedness as well as their respective subfacets (15 subfacets overall). To assess state ASC, students completed three items after each lesson in four different subjects (resulting in M[subscript lessons] = 21.32). We ran six mixed-effects location scale models: one with all broad Big Five domains and five (one for each Big Five domain) with the subfacets as predictors of state ASC. Higher scores in the domains and in at least one subfacet of open-mindedness, conscientiousness, and extraversion but lower scores in negative emotionality were related to higher mean levels of state ASC. Higher scores in depression (subfacet of negative emotionality) were related to greater within-person variability in state ASC. These findings suggest that Big Five traits are predictors of mean level and within-person variability in students' state ASC, thus contributing to a more complete map of the formation of students' ASC and the role of personality therein. Theoretical and practical implications of these findings are discussed.