Minamiite, (Na0.36K0.1Ca0.27☐0.27)Al3(SO4)2(OH)6, a new mineral of alunite group, is trigonal with a = 6.981(2), c = 33.490(14)Å R3m and Z = 6. The structure has been determined from three-dimensional X-ray diffractometer data and refined by the least-squares method to an R value of 0.045 for 389 observed reflections. The structure is composed of Al octahedra, S tetrahedra and M coordination polyhedra, and is analogous to that of alunite. Minamiite shows superlattice reflections doubling the c axis compared with that of alunite and probably caused by the partial ordering of the cations located in the M sites. The populations of M(1) and M(2) sites are determined to be Na0.44(3)K0.2☐0.36 and Na0.28Ca0.54☐0.18 respectively.