This study examined the effect of students’ externalizing problems on changes in values that they attach to math across the transition from primary to lower secondary school. Data pertaining to externalizing problems and to intrinsic, attainment, and utility values in math were gathered using the self-ratings of students in Grades 6 and 7. The analysis involved a comparison between students who reported persistent high externalizing problems before and after the transition (n = 63; 59% boys) and those who had low or non-existent externalizing problems before and after the transition (n = 1352; 50% boys). The results of a mixed-design analysis of covariance (ANCOVA) showed uniformly that students with high externalizing problems had lower intrinsic, attainment, and utility values in math than students with low or no externalizing problems. As the students progressed across the lower secondary transition, the attainment value in math showed a steeper decreasing trend in students with high levels of externalizing problems compared to the declining trajectory of students without such problems. We also found that the utility value decreased across transition, but the declining trend was steeper for students manifesting a high level of externalizing problems, particularly for boys. Overall, the results provide a better understanding of developmental trajectories of achievement values during the transition from primary to lower secondary school. Declining trends in achievement values are pronounced among students with a high level of externalizing problems, and boys with such problems are the most vulnerable group of students.