The purpose of this research was to investigate the relationship between eighth-grade students' maths-related self-concepts and their achievements in the Trends in International Mathematics and Science Study (TIMSS) 2007. The students' maths self-concept was compared with other variables, namely their mothers' education, their fathers' education and the amount of books held in the students' households. To investigate the potential role of culture, a comparison between American and Japanese students' scores was also included. The findings demonstrated that students' maths self-concept was positively associated with their achievement both in the United States and Japan, but Japanese students, while having higher achievement, had a lower maths self-concept than American students. Maternal education, paternal education and the number of books at home were also found to be positively related to achievement. Examining standardised coefficients, the study confirmed a strong interaction between maths self-concept and country, as well as a positive connection between maths self-concept as such (irrespective of the country) with maths achievement.