Dispersal is a critically important life history trait of social organisms that has a major impact on the population genetic structure and social relationships within groups. Primates exhibit highly diversified dispersal and philopatry patterns, but knowledge of these patterns is difficult to obtain and usually limited to observations of a small number of focal social groups or individuals. Here, we investigated the dispersal pattern of a critically endangered colobine monkey, the white‐headed langur (Trachypithecus leucocephalus), using molecular approaches, and sex‐specific population genetic structure analyses at fine geographical scales. We non‐invasively collected 403 fecal samples from 41 social groups across 90% of the langur's range in Fusui (FS) and Chongzuo (CZ) in southwestern Guangxi Province, China. We identified 214 unique individuals from the samples by genotyping 15 polymorphic autosomal microsatellite loci, a sex‐specific marker, and sequencing the mitochondrial DNA (mtDNA) hypervariable region I (HVRI). We found higher intragroup than intergroup genetic relatedness in males and females in both populations. A significant positive correlation between genetic distance and geographical distance, that is a pattern of isolation‐by‐distance, was detected in females from the FS population, but not in males. Spatial autocorrelation analyses revealed high within‐group relatedness in both sexes and populations, as well as an additional positive correlation at the 0.5‐km distance class in females from the FS population. Furthermore, we inferred first‐generation migrants using genetic assignment tests. Our results suggest that male T. leucocephalus disperse at random distances within habitat areas, whereas dispersal of females may mainly occur among adjacent groups near their home site. Our study provides the first genetic evidence for sex‐biased dispersal in T. leucocephalus, which has important management and conservation implications for the species.