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A new class of single-molecule magnets (SMMs) based on Dy-oxide clusterfullerenes is synthesized. Three isomers of Dy 2 O@C 82 with C s (6), C 3v (8), and C 2v (9) cage symmetries are characterized by single-crystal X-ray diffraction, which shows that the endohedral Dy-(µ 2 -O)-Dy cluster has bent shape with very short Dy-O bonds. Dy 2 O@C 82 isomers show SMM behavior with broad magnetic hysteresis, but the temperature and magnetization relaxation depend strongly on the fullerene cage. The short Dy-O distances and the large negative charge of the oxide ion in Dy 2 O@C 82 result in the very strong magnetic anisotropy of Dy ions. Their magnetic moments are aligned along the Dy-O bonds and are antiferromagnetically (AFM) coupled. At low temperatures, relaxation of magnetization in Dy 2 O@C 82 proceeds via the ferromagnetically (FM)-coupled excited state, giving Arrhenius behavior with the effective barriers equal to the AFM-FM energy difference. The AFM-FM energy differences of 5.4-12.9 cm -1 in Dy 2 O@C 82 are considerably larger than in SMMs with {Dy 2 O 2 } bridges, and the Dy∙∙∙Dy exchange coupling in Dy 2 O@C 82 is the strongest among all dinuclear Dy SMMs with diamagnetic bridges. Dy-oxide clusterfullerenes provide a playground for the further tuning of molecular magnetism via variation of the size and shape of the fullerene cage., Competing Interests: The authors declare no conflict of interest., (© 2019 The Authors. Published by WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim.)