Abstract The brittle star Ophiothrix suensonii is a com- mon coral reef sponge commensal with high dispersal potential. Here, we utilize COI sequence data from 264 O. suensonii individuals collected from 10 locations through- out Florida and the Caribbean to investigate dispersal dynamics and demographic history. Locations separated by up to 1,700 km lacked genetic differentiation, confirming the ability for long-range dispersal. However, significant differentiation was detected among other regions. Samples from Utila, Honduras showed the greatest differentiation, suggesting that the circulation of the Mesoamerican gyre could be a significant factor restricting gene flow in this region. Demographic analyses provided strong evidence for a population expansion, possibly out of Florida, through the Caribbean, and into Honduras, which commenced in the early Pleistocene. However, the presence of a clade of rare haplotypes, which split much earlier (mid-Pliocene), indi- cates that O. suensonii persisted long before its recent expansion, suggesting a cyclic history of population con- traction and expansion. Finally, patterns of gene flow are not concordant with contemporary surface currents; rather, they reflect historical movements possibly linked with changes in circulation during periods of Pleistocene climate change.