Moral justifications work, when they do, by invoking values that are shared in the common ground of the interlocutors. We asked 3- and 5-year-old peer dyads (N = 144) to identify and punish norm transgressors. In the moral condition, the transgressor violated a moral norm (e.g., by stealing); in the social rules condition, she/he violated a context-specific rule (e.g., by placing a yellow toy in a green box, instead of a yellow box). Children in both age groups justified their punishment in the social rules condition mostly by referring to the rule (e.g., “He must put yellow toys in the yellow box”). In contrast, in the moral condition they mostly justified their punishment by simply referring to the observed fact (e.g., “He stole”), seeing no need to state the norm involved (e.g., “He must not steal”), presumably because they assumed this as part of their moral common ground with their partner. These results suggest that preschoolers assume certain common ground moral values with their peers and use these in formulating explicit moral judgments and justifications.