Single-chain magnets (SCMs) are materials composed of magnetically isolated one-dimensional (1D) units exhibiting slow relaxation of magnetization. The occurrence of SCM behavior requires the fulfillment of stringent conditions for exchange and anisotropy interactions. Herein, we report the synthesis, the structure, and the magnetic characterization of the first actinide-containing SCM. The 5f-3d heterometallic 1D chains [[UO2(salen)(py)]M(py) 4]n, (M=Cd (1) and M=Mn (2); py=pyridine) are assembled trough cation-cation interaction from the reaction of the uranyl(V) complex [UO2(salen)py][Cp2Co] (Cp=pentamethylcyclopentadienyl) with Cd(NO3)2 or Mn(NO3)2 in pyridine. The infinite UMn chain displays a high relaxation barrier of 134±0.8 K (93±0.5 cm-1), probably as a result of strong intra-chain magnetic interactions combined with the high Ising anisotropy of the uranyl(V) dioxo group. It also exhibits an open magnetic hysteresis loop at T<6 K, with an impressive coercive field of 3.4 T at 2 K. Uranium in chains: 5f-3d heterometallic 1D chains are assembled from the reaction of pentavalent uranyl and CdII or MnII. The Mn-UO2-Mn coordination polymer exhibits slow relaxation of magnetization with a high relaxation barrier, and shows an open hysteresis cycle, thus affording the first example of an actinide-based single-chain magnet. Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.