Turtles are key components of modern vertebrate faunas and it is predicted that their diversity and distributions will be affected by anthropogenic climate change. Despite this, few studies have attempted to provide baseline data on turtle taxonomic richness through time or assess their past responses to global environmental change. We used the extensive Triassic–Palaeogene (252–23 Ma) fossil record of terrestrial and freshwater turtles to investigate diversity patterns, finding substantial variation in richness through time and between continents. Subsampled turtle richness was low globally from their Triassic origin until the Late Jurassic. There is strong evidence for high richness in the earliest Cretaceous of Europe. Richness was especially high following the Cretaceous Thermal Maximum and declined in all continents by the end-Cretaceous. At the K-Pg boundary, South American richness levels changed little while North American richness increased. Subsampled North American turtle richness was very high during the earliest Palaeogene (Danian) while South American richness was low; informative data on earliest Palaeogene turtle richness are lacking elsewhere. However, the Selandian–Thanetian interval, approximately 5 million years after the K-Pg mass extinction, shows low turtle richness in Asia, Europe and South America. This suggests the occurrence of exceptional turtle richness in the post-extinction Paleocene fauna of North America is not globally representative. Richness decreased over the Eocene–Oligocene boundary in North America but increased to its greatest known level for Europe, implying very different responses to dramatic Eocene–Oligocene climatic shifts. Time series regressions suggest number of formations sampled and palaeotemperature are the primary influencers of face-value richness counts, but additional factors not tested here may also be involved.