Ardisia lammersiana (Myrsinaceae) and Psychotria leptothyrsa var. defretesiana (Rubiaceae) are described from the Conservation International Mamberamo River Expedition. The new plants are locally common understory shrubs from tributaries of the lower drainage, occurring primarily in seasonally flooded forest. Among the New Guinea species in subgen. Akosmos, Ardisia lammersiana is easily identified by its long peduncles (7.0–10.5 cm), long pedicels (13–20 mm), and white flowers. The second novelty (Psychotria leptothyrsa var. defretesiana) is distinguished by its architectural form, obtuse leaf-blades, erect panicles, and 3–4-ridged pyrenes.