Background <p>Peritoneal metastasis represents one of the most lethal clinical manifestations of gastric cancer (GC) and is closely associated with immune evasion. Extracellular vesicles (EVs) have emerged as critical mediators of tumor–immune communication; however, the molecular mechanisms by which EV-associated glycosylation regulates immune escape and metastatic dissemination remain largely undefined.</p> Methods <p>EVs derived from highly metastatic GC cells were isolated and functionally characterized in vitro and in humanized immune system mouse models. Their clinical relevance was evaluated using paired human GC specimens. Underlying mechanisms were investigated through genetic manipulation, lectin-based glycosylation assays, and immune cell co-culture experiments.</p> Results <p>We identified GALNT1 as an EV-enriched glycosyltransferase that promotes GC cell invasion and suppresses CD8⁺ T-cell activation. Mechanistically, GALNT1-dependent O-GalNAc glycosylation of RPRD1A was associated with increased RPRD1A protein abundance without altering its mRNA expression. Increased RPRD1A acted as a key downstream effector driving immune escape and peritoneal metastasis. Clinically, GALNT1 and RPRD1A were significantly upregulated in GC tissues, positively correlated with each other, associated with reduced T-cell infiltration, and predictive of poor prognosis. In humanized mouse models, EV-mediated transfer of GALNT1 accelerated peritoneal dissemination and impaired antitumor immunity, whereas disruption of the GALNT1/RPRD1A axis restored T-cell infiltration and suppressed metastatic progression.</p> Conclusions <p>These findings identify an EV-driven GALNT1/RPRD1A glycosylation axis as a previously unrecognized mechanism underlying immune escape and peritoneal metastasis in GC and support this pathway as a potential therapeutic target for advanced disease.</p> Graphical Abstract <p></p>