Morinda was previously circumscribed broadly, as a pantropical group of shrubs, trees, and lianas with terminal and pseudoaxillary (i.e., 1-sided), several-flowered, capitate inflorescences with the flowers fused by their ovaries to just barely free. Additionally these species have opposite leaves, stipules that are interpetiolar to shortly tubular, distylous flowers, tubular calyx limbs, corollas with 5-6 lobes, and fleshy multiple fruits that are generally white to yellow when ripe and are composed of the ovaries of numerous flowers. Each flower has generally 2-4 seeds, which are enclosed in pyrenes. However Razafimandimbison et al. (2009) surveyed Morinda and related genera using molecular data, and concluded that Appunia (Neotropical) and Caelospermum ("Coelospermum", southeast Asia and Pacific region) are distinct and the remaining species are grouped in two distinct clades. They treated one of these clades as Morinda s. str., and the other as Gynochthodes. In general Morinda s. str. comprises trees with hermaphroditic flowers and the corolla tube longer than the corolla lobes, while Gynochthodes generally comprises lianas with polygamous to dioecious flowers and the corolla tube shorter than the corolla lobes.
The name Morinda rigida Miq. has been used in herb. for some plants from Madagascar, which are now classified in Gynochthodes. The name M. rigida applies to plants from Indonesia, which are generally similar to the Madagascar plants but the southeast Asian plants can be separated by their acute leaf tips, and from many of the Madagascar plants by their calyx limbs ca. 1-2 mm long. Here the name Morinda rigida is provisionally excluded from the Madagascar flora.