Isolation of an undescribed flavonoid glycoside from the flowers of H. calyphyllus characterised as 2-(3,4-dihydroxyphenyl)-3,5,7-trihydroxy-8-{(3,4,5-trihydroxy-6-(hydroxymethyl)tetrahydro- 2H-2-pyranyl)oxy}-4H-4-chromenone, I. The structure was clearly assigned through UV, IR, 1D, 2D-NMR and MS studies. Compound I showed influential antitubercular activity (MIC = 0.05 μg/mL) against MTB H37Rv. Distinctively, compound I showed fifteen times more active than Ethambutol (MIC = 1.56 μg/mL); four times more active than Rifampicin (MIC = 0.2 μg/mL) and two-times more active than the standard drug Isoniazid (MIC = 0.1 μg/mL).