A new naphthylisoquinoline alkaloid, named dioncophylline C, was isolated from the roots of Triphyophyllum peltatum. Its complete stereostructure was established by spectroscopic, chiroptical, and chemical methods. Dioncophylline C is the first 5,1'-linked Dioncophyllaceae alkaloid; this coupling type has hitherto been found only for Ancistrocladaceae alkaloids, yet with a different stereochemistry and a higher oxygenation degree.