Children make inductive inferences about the causal properties of individual objects from a very young age. When can they infer higher-order relational properties - a task that has proven difficult for non-human primates? In two experiments, we examined 18-24-month-old infants’ relational inferences using a causal version of a relational match-to-sample task. Results suggest that by 21-24 months of age, infants are able to infer a relational causal principle from just a few observations and use this inference to guide their own subsequent actions and bring about a novel causal outcome. Findings are considered in light of recent discussion about the nature of relational and causal reasoning, and their evolutionary origins.