The metal chelator PBT2 (5,7-dichloro-2-[(dimethylamino)methyl]-8-hydroxyquinoline) acts as a terdentate ligand capable of forming binary and ternary Cu2+complexes. It was clinically trialed as an Alzheimer’s disease (AD) therapy but failed to progress beyond phase II. The β-amyloid (Aβ) peptide associated with AD was recently concluded to form a unique Cu(Aβ) complex that is inaccessible to PBT2. Herein, it is shown that the species ascribed to this binary Cu(Aβ) complex in fact corresponds to ternary Cu(PBT2)NImAβcomplexes formed by the anchoring of Cu(PBT2) on imine nitrogen (NIm) donors of His side chains. The primary site of ternary complex formation is His6, with a conditional stepwise formation constant at pH 7.4 (Kc[M−1]) of logKc= 6.4 ± 0.1, and a second site is supplied by His13 or His14 (logKc= 4.4 ± 0.1). The stability of Cu(PBT2)NImH13/14is comparable with that of the simplest Cu(PBT2)NImcomplexes involving the NImcoordination of free imidazole (logKc= 4.22 ± 0.09) and histamine (logKc= 4.00 ± 0.05). The 100-fold larger formation constant for Cu(PBT2)NImH6indicates that outer-sphere ligand–peptide interactions strongly stabilize its structure. Despite the relatively high stability of Cu(PBT2)NImH6, PBT2 is a promiscuous chelator capable of forming a ternary Cu(PBT2)NImcomplex with any ligand containing an NImdonor. These ligands include histamine, L-His, and ubiquitous His side chains of peptides and proteins in the extracellular milieu, whose combined effect should outweigh that of a single Cu(PBT2)NImH6complex regardless of its stability. We therefore conclude that PBT2 is capable of accessing Cu(Aβ) complexes with high stability but low specificity. The results have implications for future AD therapeutic strategies and understanding the role of PBT2 in the bulk transport of transition metal ions. Given the repurposing of PBT2 as a drug for breaking antibiotic resistance, ternary Cu(PBT2)NImand analogous Zn(PBT2)NImcomplexes may be relevant to its antimicrobial properties.