Jamborite was originally described with the formula (Ni2+,Ni3+,Fe)(OH)2(OH,S,H2O) from Ca' de' Ladri and Monteacuto Ragazza near Bologna, and Castelluccio di Moscheda near Modena, Italy. Re-examination of the mineral from the type localities and Rio Vesale, Sestola, Val Panaro (Emilia-Romagna, Italy), led to the discovery of a crystal suitable for study by single-crystal and powder X-ray diffraction, SEM-EDS, and Raman spectroscopy. Jamborite crystallizes in the space group Rm, with the unit-cell parameters a 3.068(4) Å, c 23.298(11) Å, and Z = 1. The structure refinement (R1 = 0.0818) showed that jamborite contains brucite-like sheets of edge-sharing octahedra (Ni2+,M3+)(O,OH)6 with a distinctive double layer of partially occupied H2O molecules between them. Raman data indicate that the sulfur is present as sulfate rather than sulfide. The new analytical data were recalculated on the basis of 1 (Ni+Ca+Co+Fe) to give the formula [(Ni2+0.902Ca2+0.002)(Co3+0.072Fe3+0.024)]Σ1.000(OH)1.884Cl0.012(H2O)0.004(SO4)0.100·0.900H2O. The sulfur occupancy was too low to be located in the refinement, but the ≈1:1 ratio of M3+:S from the chemical analysis implies that SO42− replaces OH− in the brucite sheet rather than sitting in the interlayer space. The splitting of the H2O layer allows avoidance of short SO42−···H2O distances. Thus, jamborite is not a member of the hydrotalcite supergroup. Jamborite is redefined as M2+1−xM3+x(OH)2−x(SO4)x·nH2O, where M2+ is dominantly Ni, M3+ is dominantly Co, x ≤ 1/3 and probably ≤ 1/7 (x = 0.10 for the neotype sample), and n < (1−x). The low M3+/M2+ ratio relative to honessite and hydrohonessite and high Co content may explain the rarity of jamborite as an early alteration product of millerite. The redefinition of jamborite and designation of the neotype specimen from Rio Vesale have been approved by the Commission on New Minerals, Nomenclature and Classification (CNMNC), voting proposal 14-E.