The borate mineral satimolite, which was first described in 1969 and remained poorly-studied until now, has been re-investigated (electron microprobe analysis, single-crystal and powder X-ray diffraction studies, crystal-structure determination, infrared spectroscopy) and redefined based on the novel data obtained for the holotype material from the Satimola salt dome and a recently found sample from the Chelkar salt dome, both in North Caspian Region, Western Kazakhstan. The revised idealized formula of satimolite is KNa₂(Al₅Mg₂)[B₁₂O₁₈(OH)₁₂](OH)₆Cl₄·4H₂O (Z = 3). The mineral is trigonal, space group R3m, unit-cell parameters are: a = 15.1431(8), c = 14.4558(14) A and V = 2870.8(4) A³ (Satimola) and a = 15.1406(4), c = 14.3794(9) A and V = 2854.7(2) A³ (Chelkar). The crystal system and unit-cell parameters are quite different from those reported previously. The crystal structure of the sample from Chelkar was solved based on single-crystal data (direct methods, R = 0.0814) and the structure of the holotype from Satimola was refined on a powder sample by the Rietveld method (R_p = 0.0563, R_wp = 0.0761 and R_all = 0.0667). The structure of satimolite is unique for minerals. It contains 12-membered borate rings [B₁₂O₁₈(OH)₁₂] in which BO₃ triangles alternate with BO₂(OH)₂ tetrahedra sharing common vertices, and octahedral clusters [M₇O₆(OH)₁₈] with M = Al₅Mg₂ in the ideal case, with sharing of corners between rings and clusters to form a three-dimensional heteropolyhedral framework. Each borate ring is connected with six octahedral clusters: three under the ring and three over the ring. Large ellipsoidal cages in the framework host Na and K cations, Cl anions and H₂O molecules.