Language provides rich social information about its speakers. For instance, adults and children make inferences about a speaker's social identity, geographic origins, and group membership based on her language and accent. Although infants prefer speakers of familiar languages (Kinzler, Dupoux, & Spelke, 2007), little is known about the developmental origins of humans' sensitivity to language as marker of social identity. We investigated whether 9-month-olds use the language a person speaks as an indicator of that person's likely social relationships. Infants were familiarized with videos of two people who spoke the same or different languages, and then viewed test videos of those two individuals affiliating or disengaging. Results suggest that infants expected two people who spoke the same language to be more likely to affiliate than two people who spoke different languages. Thus, infants view language as a meaningful social marker and use language to make inferences about third-party social relationships.