We used solvent extractions, SiO$_2$ column chromatographies, and HPLC to isolate from the bark of Thanakha (Hesperethusa crenulata L.) an active crystalline compound for absorbing UV-A radiation (320 to 380 nm). Analyses of low-and high-resolution FAB-MS revealed a compound, named marmesin, with a formula of $C_{14}$ H$_{14}$ O$_4$ and a molecular mass of 246. To determine its chemical structure, we conducted 300 MMz NMR analyses using various probes, $^1$H, $^{13}$ C, and DEPT $^{13}$ C. Our NMR data showed a structure of 2,3-dihydro-2(1-hydroxy-1-methylethyl)-furanocoumarin. This active compound contains UV-absorbing chromophores, an aromatic ring, a double bond at C3-C4, and a carbonyl at C2. Its λmax is 335 nm, indicating that marmesin could be commercially useful as a natural UV-A-filtering product.