Triadenum, known as marsh St. John's worts,[2] is a small genus of flowering plants in the family Hypericaceae.

The genus is characterized by opposite, blunt-tipped leaves and pink flowers with 9 stamens.

They are distributed in North America and eastern Asia.

Kew's Plants of the World Online[3] and the Database of Vascular Plants of Canada (VASCAN)[4] treat it as a junior synonym of Hypericum and the Flora of North America[1] and Flora of China[5] treat it as separate.

The situation arises from B. R. Ruhfel et al. (2011)'s genetic study describing Triadenum as subsumed under Hypericum and later genetic results disagreeing with this assessment.