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 factor that may help some lineages diversify more than others by influencing key life history traits, such as dispersal, colonisation, environmental tolerance and reproductive success. However, the extent and direction of these effects have not been assessed across the angiosperm ToL. 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, contribute significantly to the extremely uneven distribution of diversity across the ToL.