A heteroleptic bis(tributylphosphine) platinum(II)-alkynyl complex (Pt-1) showing broadband visible-light absorption was prepared. Two different visible-light-absorbing ligands, that is, ethynylated boron-dipyrromethene (BODIPY) and a functionalized naphthalene diimide (NDI) were used in the molecule. Two reference complexes, Pt-2 and Pt-3, which contain only the NDI or BODIPY ligand, respectively, were also prepared. The coordinated BODIPY ligand shows absorption at 503 nm and fluorescence at 516 nm, whereas the coordinated NDI ligand absorbs at 594 nm; the spectral overlap between the two ligands ensures intramolecular resonance energy transfer in Pt-1, with BODIPY as the singlet energy donor and NDI as the energy acceptor. The complex shows strong absorption in the region 450 nm–640 nm, with molar absorption coefficient up to 88 000 m⁻¹ cm⁻¹. Long-lived triplet excited states lifetimes were observed for Pt-1–Pt-3 (36.9 μs, 28.3 μs, and 818.6 μs, respectively). Singlet and triplet energy transfer processes were studied by the fluorescence/phosphorescence excitation spectra, steady-state and time-resolved UV/Vis absorption and luminescence spectra, as well as nanosecond time-resolved transient difference absorption spectra. A triplet-state equilibrium was observed for Pt-1. The complexes were used as triplet photosensitizers for triplet–triplet annihilation upconversion, with upconversion quantum yields up to 18.4 % being observed for Pt-1.