Background <p>Gastric signet-ring cell carcinoma (SRCC) is an aggressive gastric cancer subtype with a high propensity for peritoneal metastasis (PM). While gut microbiota is implicated in gastric carcinogenesis, its specific role in SRCC and PM remains unknown. This study aimed to identify gut microbial markers associated with SRCC and its peritoneal metastasis.</p> Methods <p>We performed 16S rRNA gene sequencing on fecal samples from a discovery cohort (22 SRCC, 80 non-SRCC gastric cancer, 40 healthy controls) and a validation cohort (24 SRCC, 40 non-SRCC, 19 healthy controls), including 10 SRCC patients with PM and 28 without PM. Microbial diversity, differential abundance, random forest (RF) modeling, and receiver operating characteristic (ROC) analysis were used to identify candidate microbial markers.</p> Results <p>SRCC patients showed increased gut microbial biodiversity. Originally defined by Linear discriminant analysis Effect Size (LEfSe), this method identified 40 differentially abundant taxa, with Fusobacteria as a top-ranked feature by random forest. The abundance of Fusobacteria was higher in SRCC vs. non-SRCC (0.059 vs. 0.005) and in signet-ring cell carcinoma with peritoneal metastasis (S-PM) vs. without peritoneal metastasis (n-S-PM) (0.019 vs. 0.003). ROC analysis showed moderate discriminatory ability for SRCC (<i>P</i> = 0.0023) but not for PM (<i>P</i> = 0.380).</p> Conclusions <p>Gut Fusobacteria abundance is associated with gastric SRCC, but its predictive value for peritoneal metastasis is limited. These findings suggest a potential auxiliary role for Fusobacteria as a non-invasive biomarker, requiring validation in larger prospective cohorts.</p>