© TÜBİTAK.Al-rich spinels were rarely reported compared to Cr-spinels, which were mostly observed in ophiolitic rocks. Pleonaste [(Mg, Fe2+)Al2O4], which is an Al-rich spinel, was observed in the ophiolitic mafic–ultramafic rocks tectonically located in the Early Cretaceous accretionary complex, at the eastern part of the Armutlu peninsula, NW Turkey. The ophiolitic mafic–ultramafic rocks have cumulate character, and most of them are represented by peridotite and pyroxenite. Pleonaste was observed in pyroxenites and gabbros of the ophiolitic rocks. Pyroxenites consist mainly of clinopyroxene + orthopyroxene + amphibole ± olivine + spinel. Gabbros are composed of clinopyroxene + orthopyroxene + amphibole + plagioclase + spinel. Pleonaste in these rocks lies parallel to the magmatic layers and is distinguished by its emerald greenish color under the microscope. Pleonastes have high Al2O3 (59.65–62.24 wt.%) and low Cr2O3 (0.05–1.32 wt.%) contents with Mg# and Fe3+# ranging from 54.23 to 59.77 and 3.83 to 4.28, respectively. Petrographical observations and the pressure–temperature (P-T) pseudosection modelling suggest that pleonaste in the mafic–ultramafic rocks from the study area crystallized during magmatic processes. Presence of amphibole and Ca-rich (An % 85–88) plagioclase in these rocks suggests that the ophiolitic rocks, located in the Early Cretaceous accretionary complex at the eastern part of Armutlu peninsula, formed from an arcrelated hydrous magma source.