Room temperature (RT) phosphorescence is observed from a naphthalimide species for the first time in the square-planar chromophore Pt(dbbpy)(C[triple bond]C-NI)(2), where NI = N-butyl-4-ethynylnaphthalimide and dbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine. The combination of static and time-resolved absorption and photoluminescence data is uniformly consistent with triplet-state photophysics localized on an appended C[triple bond]C-NI unit following excitation into the low-energy absorption bands. This molecule features rather impressive long-lifetime, high-quantum-efficiency NI-based RT phosphorescence (tau = 124 micros; Phi = 0.215) centered at 621 nm, exemplifying how the platinum acetylide linkage strongly promotes intersystem crossing in the NI subunit, representative of a class of molecules whose excited states are typically dominated by singlet fluorescence.