The Amazon lowland rainforest flora is conventionally viewed as comprising lineages that evolved in biogeographic isolation after the split of west Gondwana (ca. 100 Myr ago). Recent molecular phylogenies, however, identify immigrant lineages that arrived in South America during its period of oceanic isolation (ca. 100–3 Myr ago). Long–distance sweepstakes dispersal across oceans played an important and possibly predominant role. Stepping–stone migration from Africa and North America through hypothesized Late Cretaceous and Tertiary island chains may have facilitated immigration. An analysis of inventory plot data suggests that immigrant lineages comprise ca. 20% of both the species and individuals of an Amazon tree community in Ecuador. This is more than an order of magnitude higher than previous estimates. We also present data on the community–level similarity between South American and palaeotropical rainforests, and suggest that most taxonomic similarity derives from trans–oceanic dispersal, rather than a shared Gondwanan history.