Extracellular accumulation of amyloid-β (Aβ) peptides in the brain plays a significant role in the development of Alzheimer’s disease (AD). While the co-localization and interaction of proteins and metal ions with Aβ in extracellular milieu are established, their precise pathological associations remain unclear. Here we report the impact of Zn(II) on the anti-amyloidogenic properties of monomeric transthyretin (M-TTR), which coexists spatially with Aβ and Zn(II) in extracellular fluids. Our findings demonstrate the Zn(II)-promoted ternary complex formation involving M-TTR, Aβ, and Zn(II) as well as M-TTR’s proteolytic activity towards Aβ. These interactions decrease the inhibitory effect of M-TTR on the primary nucleation process of Aβ as well as its ability to improve cell viability upon treatment of Aβ. This study unveils the variable activities of M-TTR towards Aβ, driven by Zn(II), providing insights into how metal ions influence the entanglement of M-TTR in the Aβ-related pathology linked to AD.