Abstract In Alzheimer’s disease (AD), fibrillar amyloid-β (Aβ) peptides form senile plaques associated with activated microglia. Recent studies have indicated that microglial Aβ clearance is facilitated by several activators such as transforming growth factor-β1 (TGF-β1). The relationship between microglia and Aβ formation and deposition is still unclear. In the present study, high mobility group protein-1 (HMG1) inhibited the microglial uptake of Aβ (1–42) in the presence and absence of TGF-β1. In addition, HMG1 bound to Aβ (1–42) and stabilized the oligomerization. In AD brains, protein levels of HMG1 were significantly increased in both the cytosolic and particulate fractions, and HMG1 and Aβ were colocalized in senile plaques associated with microglia. These results suggest that HMG1 may regulate the homeostasis of extracellular Aβ (1–42) and Aβ oligomerization.