Microtubules are highly dynamic structures, composed of / tubulin heterodimers. Biosynthesis of the functional dimer involves the participation of several chaperones, termed cofactors A-E, that act on folding intermediates downstream of the cytosolic chaperonin CCT. We show that cofactor D is also a centrosomal protein and that over-expression of either the full length protein or either of two centrosome localization domains leads to the loss of anchoring of the -tubulin ring complex and of nucleation of microtubule growth at centrosomes. In contrast, depletion of cofactor D by siRNA results in mitotic spindle defects. Because none of these changes in cofactor D activity produced a change in the levels of - or -tubulin we conclude that these newly discovered functions for cofactor D are distinct from its previously described role in tubulin folding. Thus, we describe a new role for cofactor D at centrosomes that is important to its function in polymerization of tubulin and organization of the mitotic spindle.