A family’s SES can be changeable over time. This study was the first to investigate if such within-individual changes in family SES are associated with parallel fluctuations in boys’ delinquent behavior from childhood to adolescence. Participants were a community sample of boys and their caregivers (N = 503) who were assessed annually for ten consecutive years spanning ages 7–18. Fixed effects models revealed that changes in familial SES were related to changes in delinquency: Youths were more likely to offend during years in which their parents’ SES was lower than during years in which their parents’ SES was higher. Contrary to expectations, we found no evidence that this association was accounted for by families moving to different neighborhoods or by changes in parenting. Since within-individual models provide a stricter test of causality than between-individual models, these findings support claims that impacting familial SES may have a direct effect on youths’ delinquency.