A new monostromatic species of Rhipiliopsis, a papuensis, is described from shaded subtidal habitats on the south coast of Papua New Guinea. Closely related to R. novae-caledoniae Farghaly et Denizot, it differs in having a basally corticated stipe, a lack of wall thickenings above the dichotomies of the blade siphons, and by its very regularly sinuous siphons that are linked laterally by evenly spaced cohesion papillae. New distribution records are made of R. gracilis Kraft, from the same locality as R. papuensis, and of R. echinocaulos (Cribb) Farghaly and R. madagascariensis Farghaly et Denizot from the Seychelles Islands in the western Indian Ocean. A key to the monostromatic species of Rhipiliopsis is presented.