A new complex iridoid, prismatomerin (1), has been isolated from the leaves of Prismatomeris tetrandra, together with the known glucoside gaertneroside (4). The structures of 1 and 4 were determined by spectroscopic analysis, notably 2D NMR techniques. The (1R,5S,8S,9S,10S)-(-) absolute configuration of prismatomerin (1) was determined by comparison of the vibrational circular dichroism (VCD) spectrum calculated using density functional theory and the experimental VCD spectrum of the O-acetyl derivative 3. Prismatomerin (1) showed remarkable antitumor activity and also interfered with mitotic spindle formation.