Parental monitoring impacts adolescent delinquency both directly by limiting unsupervised activities and indirectly by limiting access to delinquent peers. Deviant peers may influence adolescent delinquency through a number of mechanisms, and there is a lack of clarity within the literature on distinctions between co-offending and deviant peer norms as influential mechanisms. Less is known about the impact of co-offending on the mediated relationship among parental monitoring, peer delinquency, and adolescent delinquency. The current study examined the relationship between parental monitoring, deviant peer behaviors, co-offending, and self-reported delinquency among 186 court-involved youth (12–18 years old) in a small city in the Midwest. The effects of parental monitoring on delinquency were partially mediated by delinquent peer affiliation. A moderated mediation model found that co-offending moderated the association between delinquent peer affiliation and delinquency, such that the relationship between peer delinquency and self-reported delinquency is stronger for those who co-offend.