Shell oval globose, with an erect apex, white, somewhat solid and slightly translucent. Protoconch with 1,5 whorls. The nucleus has a diameter of 0.08 mm, with a microscopically roughened surface that extends for a little more than a half whorl. This sculpture is then replaced by several fine spiral threads, with intermediate axial extensions, that do not reach the thread above. Colour white. The teleoconch of the holotype has 2,5 whorls with rapid development; it is sculptured by elevated axial ribs which are crossed by many small spiral threads; the axial ribs number 22 on the first whorl and about 53 on the body whorl. Well defined nodules are formed at the points of intersection of the spiral and axial sculpture. Magnification reveals very fine spiral microsculpture on and between the spiral threads. Aperture ovoid, wide, with the outer lip a little angulated towards the periphery. Previous whorls are visible in the large umbilicus. Radula with the rachidian tooth showing three small denticles on each basal support; the lateral teeth are trapezoid in form, wide basally, with the sharp area also wide, where there is a central gross and blunt denticle. There are 8-9 smaller denticles on its external border and 4-5 on the internal one. Marginal internal teeth oar-shaped, strongly curved and pointed, with their external margin smooth and slight incisions in their internal margin.

The morphology of the shell and radula confirms that this species belongs to the genus Macromphalina. It can be separated from Megalomphalus pilsbryi by its lack of an umbilical keel. The teleoconch is more elevated than in most of the species described in the genus Macromphalina from Caribbean waters. Furthermore, its protoconch has a sculpture very different from that of the other species in this genus, with those of Megalomphalus pilsbryi and M. oxychone being the most similar. However, the spiral lines of M. diazmertanoi are finer, with the axial extensions longer and fewer than those on M. pilsbryi and also fewer than on M. oxychone.