Werdingite, (Mg0.84, Fe0.16)2Al12(Al0.79Fe0.21)2Si4B2(B0.77Al0.23)2O37, is a borosilicate mineral newly recorded from Namaqualand, South Africa, which has a structure based on chains of AlO6 octahedra parallel to c, cross-linked by Si2O7, groups, Al and Mg in five coordination, Al and Fe tetrahedra, and B triangles. It is triclinic P1, with a = 7.995(2), b = 8.152(1), c = 11.406(4) Å, α = 110.45(2), β = 110.85(2), γ = 84.66(2)°, V = 650.5(3) Å3, Z = 1. The structure was solved by direct methods and refined to R = 0.044 for 3584 reflections (Irel > 2σIrel) and 132 parameters. Although werdingite contains five-coordinated sites superficially similar to those in grandidierite and andalusite, the structure is best understood as being based on that of sillimanite, to which it is related by the overall substitution 2B + (Mg,Fe) = 2Si + Al + 1.5(O) with local atomic displacements resulring from the loss of nonchain O atoms. The werdingite structure provides an explanation for the mechanism of B incorporation in sillimanite, which follows the same overall substitution, and sheds light on the nature of other mullitelike borosilicate phases encountered in experimental studies of the system MgO-Al2O3-B2O3-SiO2-H2O.