The morphological ontogeny of Eobrachychthonius oudemansi van der Hammen, 1952 is investigated. This species has a dichoid body shape and several primitive characters, such as 16 pairs of notogastral setae, two transverse scissures on the notogaster and a preanal segment in the tritonymph and adult. It also has one pair of exobothridial setae, three pairs of adanal setae and two pairs of anal setae. The adults have light spots on the prodorsum and notogaster, and two pleural and six suprapleural plates, which are absent in the juveniles. In all instars of E. oudemansi, two transverse scissures are present on the notogaster, dividing it into three plates: NA, NM and PY. The larva has 14 pairs of notogastral setae, protonymph has 17 pairs, and other nymphs 16 pairs; most are of medium size, curved and smooth. In all instars, the gastronotal plate NA has six pairs of setae (c-and d-series), plate NM has two pairs (e-series), whereas the number of setae on the pygidium (PY) increases during ontogeny, from four pairs in the larva (f-series, h₁, h₂) to eight pairs (f₁–f₂, h₁–h₃, p₁–p₃) in the nymphs and adult. The morphology of E. oudemansi is compared with congeners.