Aim: To quantify taxon diversity, biogeographic distributions and patterns of community assembly in xeniid octocorals using molecular operational taxonomic units (MOTUs). Location Red Sea, Indian and western Pacific Oceans. Taxon Xeniidae, a family of reef-dwelling octocorals (Anthozoa, Octocorallia). Methods Xeniids collected at 13 locations were sequenced at three barcode loci, and assigned to molecular operational taxonomic units (MOTUs) defined by minimum genetic distance thresholds. Taxon richness (no. of MOTUs) and endemicity (percent of MOTUs found at a single location) were quantified. Patterns of β-diversity (species turnover) and phylogenetic β-diversity (lineage turnover) among geographical regions were visualized using hierarchical clustering, NMDS plots, and distance-decay relationships. Community assembly was investigated by comparing the mean pairwise distance (MPD) and mean nearest taxon distance (MNTD) separating species in each assemblage to values generated for null communities. Results A genetic distance threshold of 0.3% discriminated 67 MOTUs, with taxon richness ranging from 2-18 MOTUs per site. Out of the 67 MOTUs, 48 (72%) were found at only a single location, and only two spanned both the western Indian and Pacific Oceans. Species turnover among sites was high, but phylogenetic β-diversity was lower than β-diversity and differed significantly from null models of community assembly at only two sites. β-diversity and phylogenetic β-diversity both increased significantly with geographic distance, and sites clustered into three distinct biogeographic regions (Red Sea and western Indian Ocean; Western Australia; western Pacific and Great Barrier Reef, Australia). All five major clades of xeniids were represented in each region. Main Conclusions A genetic approach to biodiversity estimation suggests that most xeniid taxa are regional endemics whose geographic distribution is likely governed by dispersal limitation. This conclusion contrasts with published records of certain morphospecies occurrences, which imply that they have broad geographic ranges. So far, the distribution of xeniid biodiversity mirrors that of scleractinian corals, with species richness highest in the Coral Triangle, but endemicity peaking in peripheral areas.