Marmelosin is a naturally occurring coumarin derivative primarily isolated from the fruit pulp and leaves of Aegle marmelos (bael fruit). It belongs to the class of methoxylated coumarins substituted with an isovaleryl side chain. This compound contributes to the characteristic aroma and reported pharmacological activities of bael, including antimicrobial, anti-inflammatory, and antidiabetic effects observed in traditional medicine. Structurally, it features a benzopyrone core with a methoxy group at position 7 and a 3-methyl-2-oxobutyl substituent at position 6. It is lipophilic, consistent with other plant coumarins, and occurs in low concentrations in edible portions of the fruit.