One of the hallmarks of Alzheimer's disease is the formation of aggregates of the tau protein, a process that can be facilitated by the presence of fibrils formed by the amyloid β peptide (Aβ). However, the mechanism that triggers tau aggregation is still a matter of debate. The effect of Aβ40 fibrils on the aggregation of the repeat domain of tau (TauRD) is investigated here by employing coarse-grained molecular dynamics simulations. The results indicate that the repeat domain of tau has a high affinity for Aβ40 fibrils, with the 261GSTENLK267 fragment of tau driving TauRD toward the 16KLVFFA21 fragment in Aβ40. Monomeric Aβ40, in which the 16KLVFFA21 fragment is rarely found in an extended conformation (as in the fibril), has a low affinity for the TauRD, indicating that the ability of Aβ40 fibrils to bind to the TauRD depends on the 16KLVFFA21 fragment of Aβ adopting an extended conformation.