Cyclometalated iridium(III) complexes with intense absorption in the visible region (ϵ = 70920 M–1 cm–1 at 466 nm) were prepared by incorporating light-harvesting coumarin into the diimine ligand (for comparison: ϵ = 7030 M–1 cm–1 at 371 nm for the model diimine complex without light-harvesting ability). The complexes feature long-lived intraligand triplet excited states (3IL, supported by spin density analysis of the triplet state). At room temperature, lifetimes, τT, are up to 75.5 μs, as determined by nanosecond time-resolved transient absorption spectroscopy. Despite the weak phosphorescence (ΦP = 0.6–0.7 %) observed for the complexes, the triplet excited state (T1) was demonstrated to be efficiently populated upon photoexcitation, by using the complexes as triplet sensitizers in triplet–triplet-annihilation (TTA) upconversion (upconversion quantum yields, ΦUC, are up to 23.4 %). We propose that weakly phosphorescent or even nonphosphorescent transition metal complexes can be used as triplet sensitizers for TTA upconversion, instead of the currently used phosphorescent complexes. Our design of the light-harvesting IrIII complexes will be helpful for the preparation of transition metal complexes with intense absorption of visible light and long-lived triplet excited states, which are suitable for applications in areas such as photocatalysis, photovoltaics, and upconversion.