Species diversity varies greatly across the different taxonomic groups that comprise the Tree of Life (ToL). This imbalance is particularly conspicuous within angiosperms, but is largely unexplained. Seed mass is one trait that may help explain why some lineages diversify more than others because it integrates across many key life history traits that influence speciation and extinction, such as dispersal, survival, environmental tolerance and reproductive success. However, the extent and direction of the association between seed mass and diversification has not been assessed across the angiosperm phylogeny. Here, we show for the first time that absolute seed size and the rate of change in seed size are both associated with variation in diversification rates. Based on an unequalled phylogenetic tree that included 4105 angiosperm genera, we found that smaller-seeded plants had higher rates of diversification, possibly due to improved colonisation potential. The rate of phenotypic change in seed size was also strongly positively correlated with speciation rates, supporting emerging evidence that rapid morphological change is associated with species divergence. Our study now reveals that variation in morphological traits, as well as the rate at which traits evolve, can contribute to explaining the extremely uneven distribution of diversity across the ToL.