In this study we describe and illustrate the morphological ontogeny of a new ceratozetid oribatid mite, Sphaerozetes olympicussp. nov. from moss on the rocky soil in a pine forest on the slope of Olympus Mtn. (Greece). The adult of S. olympicus is similar to that of Sphaerozetes orbicularis (C.L. Koch, 1835), but has a rostrum with two shallow incisions, a short lamellar cusp, most notogastral setae vestigial (alveolar) and seta p₁ longer than others of the p-series; in S. orbicularis the rostrum has two deep incisions, the lamellar cusp is of medium size, most notogastral setae are distinguishable (though minute) and p₁ is as short as other setae of the p-series. The juveniles of both species have most gastronotal setae short, but S. olympicus has seta h₁ distinctly longer and thicker than other gastronotal setae, whereas in S. orbicularis this seta is short. Moreover, in S. olympicus the prodorsal seta le is thicker than in and blunt, whereas in S. orbicularis le is as thin as in and acuminate. The juveniles of S. olympicus also have legs and claws I and II thicker than those of S. orbicularis. The larva of S. olympicus lacks a humeral organ and a humeral macrosclerite, which is unique among Sphaerozetes. The nymphs of this species have a humeral macrosclerite, but it is small and glabrous; moreover, seta c₁ is inserted on a microsclerite, which is also unique in Sphaerozetes. Nymphs of S. orbicularis have a humeral macrosclerite distinctly larger than in S. olympicus, and it bears seta c₁. We compare the morphological ontogeny of S. olympicus with that of S. orbicularis, discuss the morphology of Sphaerozetes adults, with a focus on the shape of the rostrum and pattern of notogastral setae, which are important diagnostic characters of Sphaerozetes, and modify the diagnosis of this genus.