What leads children to moralize actions that cause no apparent harm? We hypothesized that adults’ verbal instruction (“testimony”), as well as emotions such as disgust, would influence children’s moralization of apparently harmless actions. To test this hypothesis, 7-year-old children were asked to render moral judgments of novel, seemingly victimless, body-directed or nature-directed actions after being exposed to adults’ testimony or to an emotional induction. Study 1 demonstrated that children became more likely to judge actions as “wrong” upon being verbally presented with testimony about disgust or anger—but not upon being directly induced to feel disgusted. Study 2 established that principle-based testimony is an even more powerful source of moralization, and additionally found long-term retention of newly formed moral beliefs. These studies also indicated that children frequently lack introspective insight into the sources of their newly acquired moral reactions; they often invoked welfare-based concerns in their explanations regardless of experimental condition. In sum, this research demonstrates that children rapidly and enduringly moralize entirely unfamiliar, apparently innocuous actions upon exposure to a diverse array of morally relevant testimony.