The emergence of moral emotions such as guilt is central in moral and prosocial development. Guilt is an important psychological factor, which motivates prosocial behaviour and is credited for multiple social functions. Importantly, it remains unclear what determines the extent to which children show guilt. The current study examined two factors that have been proposed to support the emergence of guilt: children's moral self-concept (MSC) and Theory of Mind (ToM). To this end, guilt-related behaviour and the two potentially explanatory factors were assessed in 5-6-year-old children (Mean age: 5.43 years, N = 133, 51% female). Most relevant, MSC predicted children's guilt-related behaviour. Children's ToM was not predictive for guilt-related behaviour. These findings support theoretical approaches proposing that the emergence of guilt is based on children's developing self-concept.