In marine environments, paleoceanographic changes can act as drivers of diversification and speciation, even in highly mobile marine organisms. Shearwaters are a group of pelagic seabirds with a well-resolved phylogeny that are globally distributed and show periods of both slow and rapid diversification. Using reduced representation sequencing data, we explored the role of paleoceanographic changes on diversification and speciation in these highly mobile pelagic seabirds. We performed molecular dating, applying a multispecies coalescent approach (MSC) to account for the high levels of incomplete lineage sorting (ILS). We identified a major effect of the Pliocene marine megafauna extinction, followed by a period of high dispersal and rapid speciation. Biogeographic analyses showed that dispersal appears to be favoured by surface ocean currents, and we found that founder and vicariant events are the main processes of diversification. Body mass appears to be a key phenotypic trait potentially under selection during shearwater diversification, and it shows significant associations with life strategies and local conditions. We also found incongruences between the current taxonomy and patterns of genomic divergence, suggesting revisions to alpha taxonomy. Globally, our findings extend our understanding on the drivers of speciation and dispersal of highly mobile pelagic seabirds and shed new light on the important role of paleoceanographic events.