The crystal structure of bijvoetite-(Y), [ M8 3+ (H2O)25(UO2)16O8(OH)8(CO3)16](H2O)14, M = (Y, REE), pseudo-orthorhombic, monoclinic, a 21.234(3), b 12.958(2), c 44.911(6) A, b 90.00(2)°, V 12,357(6) A 3 , space group B1211, Z = 4, was solved by direct methods and refined by least-squares techniques to an agreement index ( R) of 8.4% and a goodness of fit (S) of 0.81 for 6622 unique observed (|Fo| > 4sF) reflections collected for a twinned crystal using graphite-monochromatic Mo Ka X-radiation and a CCD area detector. There are 16 symmetrically independent U 6+ positions, each of which is part of near-linear (U 6+ O2) 2+ uranyl ion. Eight uranyl ions are coordinated by three O 2‐ and two (OH) ‐ anions each, resulting in uranyl pentagonal bipyramids, and eight uranyl ions are coordinated by six O 2‐ anions each, forming uranyl hexagonal bipyramids. The structure contains 16 unique carbonate groups and eight unique M 3+ sites that are occupied by variable amounts of Y, Dy and other REEs. The structure of bijvoetite-(Y) is based on a novel uranyl carbonate chain that is parallel to [100], involving edge-sharing dimers of uranyl pe ntagonal bipyramids, edge-sharing dimers of uranyl hexagonal bipyramids, and carbonate groups. The chains are cross-linked by irregular M 3+ fn (f: unspecified ligand) polyhedra, forming (Y, REE)-bearing uranyl carbonate sheets that are parallel to (010). The sheet in bijvoetite-(Y) is based upon a new sheet anion-topology. There are 39 symmetrically unique H 2O groups in the structure, 25 of which are bonded to M 3+ and 14 of which are located in the interlayer, where they are held in place by H bonds. The interlayer of the structure contains only H 2O groups, and adjacent sheets are connected by H bonds only. The discovery of the structure may have implications for nuclear waste disposal because uranyl carbonates may be abundant in a geological repository due to the corrosion of uranium oxide nuclear fuel. It is proposed that the structure may incorporate transuranium elements.