A methanol extract of the bark of Halfordia kendak yielded a compound named halfordinin in addition to the known 3,4,5- and 3,4,8-trimethoxylated linear furanocoumarins halfordin (I) and isohalfordin (IV). Microanalysis (establishing composition C₁₈H₁₈O₆), UV, IR, PMR, mass spectrometry, hydrogenation (giving a tetrahydro derivative), and acid treatment (yielding norhalfordin identical to that from acid-catalyzed demethylation of halfordin) were used to characterize halfordinin. The results showed that halfordinin is a 3,4,5-oxygenated linear furanocoumarin with a novel α,α-dimethylallyl ether substituent at the 3-position (structure III). Biogenetic points included that it is the third natural coumarin oxygenated at the 3- and 4-positions, the first naturally occurring compound with an α,α-dimethylallyl ether moiety, and its C₅ sidechain arises from direct alkylation of the 3-hydroxy group of the coumarin ring by γ,γ-dimethylallyl pyrophosphate.