The structure of agrinierite, K2(Ca0.65Sr0.35)[(UO2)3O3(OH)2]2·5H2O, orthorhombic, F2mm, Z = 16, a = 14.094(2), b = 14.127(2), c = 24.106(4) Å, V = 4799.6(1) Å3, was solved by direct methods and refined by full-matrix least-squares techniques to an agreement factor (R) of 6.55% and a goodness-of-fit (S) of 0.851 using 2710 independent observed reflections collected with MoKα X-radiation and a CCD-based detector. This layered material contains four unique U6+ positions, each of which is part of a nearly linear (UO2)2+ uranyl ion. The U6+ cations are further coordinated by five anions occupying the equatorial vertices of pentagonal bipyramids that are capped by the uranyl ion O atoms. The uranyl polyhedra are linked by the sharing of equatorial vertices and edges in a fashion topologically identical to the α-U3O8 sheet found in billietite, protasite, becquerelite, richetite, compreignacite and masuyite. The arrangement of hydroxyl anions within the sheets varies in these minerals; that of agrinierite is identical to protasite. The cations (Ca, Sr, and K) and H2O reside in the interlayer region of the structure. The inclusion of Sr in the structure of agrinierite suggests that the release of radioactive 90Sr may be impacted by incorporation into this phase if it forms in a geological repository for nuclear waste.