Abnormal deposition of β-amyloid (Aβ) is a core pathological feature of Alzheimer's disease (AD). Syndecan-3 (SDC3), a type I transmembrane heparan sulfate proteoglycan (HSPG), is abnormally overexpressed in the brains of AD patients and model animals, specifically accumulating in the peri-plaque region of amyloid plaques. However, its regulatory mechanism in the process of Aβ deposition remains unclear. This study aims to clearly define the role of SDC3 in Aβ aggregation and neuroinflammation, two critical processes in AD pathogenesis. Specifically, we investigate how SDC3 modulates Aβ aggregation and its interaction with neuroinflammatory pathways, which may contribute to the progression of AD. By elucidating the mechanisms underlying SDC3's involvement in these processes, we seek to provide new insights into potential therapeutic targets for AD. In this study, a 5×FAD mouse model with downregulated SDC3 expression was constructed. Behavioral assessments and synaptic function tests were performed to explore the effects of SDC3 on cognition in 5×FAD mice. Immunofluorescence co-localization technology was utilized to analyze the pathological co-deposition of SDC3 and Aβ in the hippocampus, cortex, and meningeal blood vessels. Quantitative assessments of pro-inflammatory cytokines such as and in the brain were performed through histopathological analysis combined with qPCR. Western blotting was used to examine the phosphorylation status of STAT1/STAT3 and the expression changes of IBA1/GFAP to systematically analyze the molecular mechanisms through which SDC3 regulates AD pathology. This study revealed that SDC3 expression was significantly upregulated in the brain regions of the 5×FAD model mice and co-localized pathologically with Aβ. Cell lineage tracing analysis showed that the elevated SDC3 expression primarily originated from glial cells. Behavioral and pathological results demonstrated that downregulation of SDC3 significantly improved cognitive dysfunction in the model mice and effectively reduced the Aβ burden in the brain. Molecular mechanism studies showed that downregulation of SDC3 reduced the phosphorylation of STAT1 and STAT3, thereby inhibiting the activation of the JAK-STAT and cGAS-STING signaling pathways, reducing the activation of microglia/astrocytes and suppressing the expression of pro-inflammatory cytokines such as and . This study reveals that SDC3 co-localizes with Aβ pathology and synergistically exacerbates neuroinflammation. Knockdown of SDC3 can simultaneously reduce both Aβ deposition and the release of inflammatory factors from glial cells. Mechanistic research indicates that SDC3 drives a "glial activation-cytokine release" vicious cycle through the JAK-STAT and cGAS-STING signaling pathways. These findings suggest that SDC3 may serve as a key hub coordinating amyloid pathology and neuroinflammation in AD, providing new insights for the development of combination therapies targeting the HSPG network.