Incremental predictive value of 5 broad and 13 narrow personality traits for academic achievement over and beyond age, gender, parental education, and country was examined in Russian and Slovene 8- to 15-year-olds. Personality data were collected from mothers (Russia: N = 994, Slovenia: N = 624) and adolescents (Russia: N = 481, Slovenia: N = 310) using the Inventory of Child Individual Differences–Short. Final grades in mathematics and Russian/Slovene language were considered as measures of academic success. The broad and the narrow traits predicted the grades, improving the criteria variance by 11% to 14% and 15% to 19%, respectively, over the background variables. Across the countries, informants and school subjects, younger age, higher parental education, female gender, openness (especially subjectively perceived intelligence), and low extraversion predicted academic achievements. Mother-assessed conscientiousness (especially low distractibility) was also predictive of school grades.