While many studies suggest that poor academic performance predicts delinquency, others have argued that the relationship between academic performance and delinquency can be marginal, null, or even spurious. This study examines the relationship between academic performance and delinquent behavior among Korean youth within the general strain framework, utilizing data from the Korean Child and Youth Panel Survey. Using a sample of 2,378 students from 95 randomly selected schools in Korea, the KCYP survey examined students’ perceptions and experiences regarding relationships with parents, teachers, and peers, as well as their engagement in various delinquent behaviors. Group-based trajectory modeling identified two academic trajectory groups: consistently low academic grades and declining academic performance. Two dichotomous variables representing these academic trajectory groups were included in the negative binomial regression models. After accounting for the effects of other strain-related and social/self-control variables, the effects of low academic performance on delinquent behavior become spurious while a decline in academic performance is a consistent predictor of delinquent behavior across all models.