We describe the development of social play behavior and assess factors influencing the development of play in infant Sichuan snub-nosed monkeys (Rhinopithecus roxellana). Infant snub-nosed monkeys began to exhibit social play at 3 months of age, when they spent an average 0.89% of time engaging in this behavior (range: 0.7–1.12%). At 6 months of age, there was a significant increase in the proportion of time spent in social play, averaging 9.78% of observation time (range: 4.92–17.08%). However, from 7 to 9 months of age during the winter, social play decreased gradually before rising again from 10 months of age in the spring. Play behavior in infant snub-nosed monkeys is influenced by environmental temperature. Males were observed to play more than females, although further data on this are required. Social rank did not influence the social play of wild Sichuan snub-nosed monkey infants. Am. J. Primatol. 73:845–851, 2011. © 2011 Wiley-Liss, Inc.