Past research has demonstrated that perceivers spontaneously infer individuals' goals, beliefs, and traits from their behaviors. These inferences processes are essential for predicting others' future behaviors and, thus, for smooth social interaction. Given that social roles (e.g., professor, mother) are also predictive of an individual's future behaviors, we proposed that perceivers spontaneously infer individuals' social roles from their behaviors. Across three experiments, including two different paradigms, we documented that perceivers formed spontaneous role inferences (SRIs) from single behaviors. SRIs occurred unintentionally, efficiently, and had important downstream consequences for impression formation. Namely, SRIs led perceivers to rate targets as higher on role-consistent traits. Together, these findings provide the first empirical demonstration of a novel process in impression formation. Published by Elsevier Inc.