Although octocorals are important components of coral reefs, most research on the genetic diversity of symbiotic zooxanthellae (Symbiodinium spp.) has focused on scleractinian (stony) corals. For both groups, most geographic comparisons have occurred within the same ocean or only included a few geographic sites. We characterized the genetic diversity of Symbiodinium in tropical octocorals in 15 geographic areas throughout the world. By combining octocoral samples analyzed in this study with published data, the cladal identities of zooxanthellae in 117 octocoral species were identified and a global analysis was performed. For 63 octocoral species, the Symbiodinium type was also identified. The vast majority (87.2%) of octocorals hosted a single zooxanthella clade. Most octo- corals also hosted specific Symbiodinium types. A biogeographic pattern emerged. Octocorals from 4 geographic areas in the Indo-west Pacific hosted Symbiodinium C. In the Great Barrier Reef, 11 octo- coral species (24.4%) hosted Symbiodinium D, 3 species hosted Symbiodinium G, and 1 species each hosted Symbiodinium A and B. Red Sea octocorals hosted predominantly Symbiodinium C, with Symbiodinium A occurring in 3 species. In Hawaii, 2 octocorals hosted Symbiodinium C and 1 hosted Symbiodinium A. In Bermuda, only clade B was found. In the Caribbean, unlike in the Indo-Pacific, the dominant Symbiodinium was B, predominantly type B1, with Symbiodinium C occurring in a few species. Understanding the similarities and differences between octocoral and scleractinian coral symbioses with zooxanthellae may enable predictions of which symbioses will survive in periods of environmental change.