Room-temperature luminescence of 7-diethylamino-4-hydroxycoumarin (16C) was investigated in solvents of varying polarity and viscosity, such as methanol, propylene glycol, and in poly(vinyl alcohol) (PVA) matrix. Experimental studies were complemented by extensive quantum-chemical calculations, including spin-orbit coupling, to elucidate the deactivation pathways of the emissive singlet to triplet excited states and to assess the influence of tautomerism on the photophysical properties of the compound. Both fluorescence and phosphorescence were observed for 16C in the PVA film, centered at around 400 and 500 nm, respectively. Interestingly, phosphorescence was highly efficient at room temperature, exhibiting a long lifetime of up to 0.764 s and ca. 3 s afterglow under naked eye observation. The pronounced emission anisotropy observed in PVA confirms strong immobilization of embedded 16C molecules. Computational studies suggest that the diketo form of 16C may intrinsically favor intersystem crossing through effective coupling between the first singlet excited state of mixed 1ππ∗/1CT character and a low-energy 3nπ∗-type triplet involving C4double bondO carbonyl group present in this tautomer, highlighting a possible tautomeric dependence of the emission in this derivative. The presented system offers a promising platform for the development of new materials exhibiting robust fluorescence and phosphorescence, governed by a proton-transfer process.