Children with conduct problems engage in more frequent antisocial lying compared to typically developing children (Zanette et al., 2020). However, little is known about why children with conduct problems tell more antisocial lies and whether their moral understanding of lying plays a role in these differences. The current study explored whether children with conduct problems differ from typically developing children in their moral understanding of truth- and lie-telling in antisocial contexts. Child participants (N = 387; 6-13 years-old) completed a moral reasoning task examining why they believe telling the truth or a lie about having committed a transgression (e.g., eating a candy without permission) is either good or bad. Responses were coded based on their rationale for these judgements (e.g., “it is bad because she might get in trouble”). Children with conduct problems did not significantly differ from typically developing children in why they believe it is good to tell the truth and confess to a misbehaviour is good. However, for males only, children with conduct problems significantly differed from typically developing children in their rationales for why they believe telling an antisocial lie is bad. Compared to typically developing males, males with conduct problems were more likely to judge telling an antisocial lie as bad based on the nature of the transgression committed (e.g., disobeying a teacher) rather than the morality of lying about said transgression. This research contributes to our limited understanding of lying and conduct problems and may help inform the development of more effective intervention strategies to combat patterns of excessive lying.