Brain accumulation of amyloid-β (Aβ) peptides (resulting from a disrupted balance between biosynthesis and clearance) occurs during the progression of Alzheimer's disease (AD). Aβ peptides have diverse posttranslational modifications (PTMs) that variously modulate Aβ aggregation into fibrils, but understanding the mechanistic roles of PTMs in these processes remains a challenge. Here, we chemically synthesized three homogeneously modified isoforms of Aβ (1-42) peptides bearing Tyr10 O-glycosylation, an unusual PTM initially identified from the cerebrospinal fluid samples of AD patients. We discovered that O-glycans significantly affect both the aggregation and degradation of Aβ42. By combining cryo-EM and various biochemical assays, we demonstrate that a Galβ1-3GalNAc modification redirects Aβ42 to form a new fibril polymorphic structure that is less stable and more vulnerable to Aβ-degrading enzymes (e.g., insulin-degrading enzyme). Thus, beyond showing how particular O-glycosylation modifications affect Aβ42 aggregation at the molecular level, our study provides powerful experimental tools to support further investigations about how PTMs affect Aβ42 fibril aggregation and AD-related neurotoxicity.