Uranium-based compounds have been put forward as ideal candidates for the design of single-molecule magnets (SMMs) with improved properties, but to date, only two examples of exchange-coupled 3d-5f SMM containing uranium have been reported and both are based on the Mn-II ion. Here we have synthesized the first examples of exchange-coupled uranium SMMs based on Fe-II and Ni-II. The SMM behavior of these complexes containing a quasi linear M-O=U=O-M core arises from intramolecular Fe-U and Ni-U exchange interactions combined with the high Ising anisotropy of the uranyl(V) moiety. The measured values of the relaxation barrier (53.9 +/- 0.9 K in the UFe2 complex and of 27.4 +/- 0.5 K in the UNi2 complex) show clearly the dependency on the spin value of the transition metal, providing important new information for the future design of improved uranium-based SMMs.