Background: Tropical rainforests (TRFs) harbour almost half of the world’s vascular plant species diversity while covering only about 6–7% of land. However, why species richness varies amongst the Earth’s major TRF regions remains poorly understood. Here we investigate the evolutionary processes shaping continental species richness disparities of the pantropical, epiphytic and mostly TRF-dwelling orchid genus Bulbophyllum (c. 1,948 spp. in total; Asia-Pacific region: c. 1,564 spp.; Madagascar: 210; Africa: 80; Neotropics: 94) using diversification analyses based on a time-calibrated molecular phylogeny, coupled with ecological niche modelling (ENM) of geographic distributions under present and past (Last Glacial Maximum) conditions. Results: Our results suggest an early-to-late Miocene scenario of ‘out-of-Asia-Pacific’ origin and progressive, dispersal-mediated diversification in Madagascar, Africa and the Neotropics, respectively. Species richness disparities amongst these four TRF lineages are best explained by a time-for-speciation effect rather than differences in net diversification or diversity-dependent diversification due to present or past spatial-bioclimatic limits. All four lineages of experienced dramatic range expansions during the LGM, which conflicts with the common notion that TRFs mostly fragmented/contracted during glacial periods. Conclusions: Most species of at least the Madagascan, African and Neotropical lineages originated during the Quaternary. Their diversification under high species turnover (i.e. high rates of speciation and extinction) might relate to climate-induced range fluctuations during this time period combined with various intrinsic features commonly invoked to foster rapid population turnover in tropical orchids (e.g., epiphytism, specialization on pollinators and mycorrhizal fungi, dispersal by wind). Further (e.g., phylogenomic and ecological) research within each Bulbophyllum lineage but also other pantropical TRF taxa is required to provide a better understanding of how evolutionary processes as well as past and current environmental conditions drive tropical biodiversity and account for regional differences in species richness patterns on a global scale.