Lobeliads (Campanulaceae: Lobelioideae) arrived on Hawai’i from a single ancestor ~15 mya. Since then, they have diversified to become the archipelago’s largest plant radiation, comprising ~12% of the native species diversity. With ~140 known species spread across 6 genera, the Hawaiian lobeliads have undergone an adaptive radiation which encompasses a panoply of growth forms, habitat types, floral forms, photosynthetic rates, and morphological leaf traits. The morphological diversification of leaves includes shape, size, margin, armature, and perhaps most notably, changes in leaf shape within the lifespan of an individual (heteroblasty). The latter is restricted to the genus Cyanea, which in itself makes up more than half of the species diversity in the clade. We quantify leaf size and shape in Hawaiian lobeliads, and examine the partitioning of morphological diversity across genera, islands, and habitat types. We also quantify disparity in juvenile and adult leaves in heteroblastic species. These results are discussed in light of new, time-calibrated phylogenetic hypotheses for the clade based on analyses of hundreds of nuclear loci and nearly complete plastomes.