Introduction. Familiar and unfamiliar face perception is typically dissociated by the relative use of internal and external face features. The Williams syndrome (WS) social phenotype emphasises hypersociability, with an interest in interacting with people irrespective of familiarity. The aim is to explore whether unfamiliar face processing is characterised by the typical dissociation between internal and external features in WS, or whether the social stimulus drive towards strangers is linked to atypicalities of unfamiliar face processing. Method. The procedure replicates that previously used with typically developing children. Participants with WS (aged 10-18 years) and typically developing comparison participants determine whether two face parts are from the same person or different people, using the whole face, internal, and external features. Results. Only participants with WS, and not typically developing participants, show greater accuracy matching unfamiliar faces from internal than external features. Conclusions. Evidence of atypical unfamiliar face processing in WS may inform models of typical face perception, revealing the origins of the relative advantage for internal features typically associated with familiar but not unfamiliar faces. The results also have implications for understanding more clearly the social phenotype associated with WS.