Phlorofucofuroeckol A (1), a novel phlorotannin with both dibenzo-1, 4-dioxin and dibenzofuran elements, has been isolated from the brown alga Ecklonia kurome OKAMURA as a potent anti-plasmin inhibitor. Its structure has been elucidated to be 1, 11-di(3, 5-dihydroxyphenoxy)-benzofuro[3, 2-a]dibenzo[b, e][1, 4]dioxin-2, 4, 8, 10, 14-pentaol on the basis of the spectral data, in particular, by means of negative nuclear Overhauser effect (NOE) and long-range carbon-proton couplings (²J<CH> and ³J<CH>). Phlorofucofuroeckol A inhibited the action of α₂-macroglobulin (IC<50>=1.0μg/ml) and α₂-plasmin inhibitor (IC<50>=0.3μg/ml), the main plasmin inhibitors in plasma.