Amyloid-beta (A beta) aggregation and neuroinflammation are consistent features in Alzheimer's disease (AD) and strong candidates for the initiation of neurodegeneration. S100B is one of the most abundant proinflammatory proteins that is chronically up-regulated in AD and is found associated with senile plaques. This recognized biomarker for brain distress may, thus, play roles in amyloid aggregation which remain to be determined. We report a novel role for the neuronal S100B protein as suppressor of A beta 42 aggregation and toxicity. We determined the structural details of the interaction between monomeric A beta 42 and S100B, which is favored by calcium binding to S100B, possibly involving conformational switching of disordered A beta 42 into an alpha-helical conformer, which locks aggregation. From nuclear magnetic resonance experiments, we show that this dynamic interaction occurs at a promiscuous peptide-binding region within the interfacial cleft of the S100B homodimer. This physical interaction is coupled to a functional role in the inhibition of A beta 42 aggregation and toxicity and is tuned by calcium binding to S100B. S100B delays the onset of A beta 42 aggregation by interacting with A beta 42 monomers inhibiting primary nucleation, and the calcium-bound state substantially affects secondary nucleation by inhibiting fibril surface-catalyzed reactions through S100B binding to growing A beta 42 oligomers and fibrils. S100B protects cells from A beta 42-mediated toxicity, rescuing cell viability and decreasing apoptosis induced by A beta 42 in cell cultures. Together, our findings suggest that molecular targeting of S100B could be translated into development of novel approaches to ameliorate AD neurodegeneration.