Alzheimer's disease is associated with the deposition of the amyloid-β peptide (Aβ) into extracellular senile plaques in the brain. In vitro and in vivo observations have indicated that transthyretin (TTR) acts as an Aβ scavenger in the brain, but the mechanism has not been fully resolved. We have monitored the aggregation process of Aβ40 by thioflavin T fluorescence, in the presence or absence of different concentrations of preformed seed aggregates of Aβ40, of wild-type tetrameric TTR (WT-TTR), and of a variant engineered to be stable as a monomer (M-TTR). Both WT-TTR and M-TTR were found to inhibit specific steps of the process of Aβ40 fibril formation, which are primary and secondary nucleations, without affecting the elongation of the resulting fibrils. Moreover, the analysis shows that both WT-TTR and M-TTR bind to Aβ40 oligomers formed in the aggregation reaction and inhibit their conversion into the shortest fibrils able to elongate. Using biophysical methods, TTR was found to change some aspects of its overall structure following such interactions with Aβ40 oligomers, as well as with oligomers of Aβ42, while maintaining its overall topology. Hence, it is likely that the predominant mechanism by which TTR exerts its protective role lies in the binding of TTR to the Aβ oligomers and in inhibiting primary and secondary nucleation processes, which limits both the toxicity of Aβ oligomers and the ability of the fibrils to proliferate.