Hereditary diffuse gastric cancer (HDGC) is a rare autosomal dominant syndrome associated with an increased risk of developing Laurén's diffuse-type gastric carcinoma and lobular breast carcinoma. Although signet-ring cell carcinoma (SRCC) in situ (SRCC-pTis) has been reported as a characteristic lesion in HDGC cases with CDH1 germline mutations (CDH1 pathogenic variant), and a precursor of conventional intramucosal SRCC (SRCC-pT1a), its histopathologic features and specificity have not been sufficiently clarified. Here, we examined gastrectomy samples from 6 Japanese HDGC patients with CDH1 germline mutation, belonging to 4 families, and analyzed SRCC lesions histologically and immunohistochemically. Of the 274 foci found in the 6 samples, SRCC-pT1a accounted for 225 lesions (range: 8 to 107, mean 45.7 lesions per patient), while 46 foci were of SRCC-pTis (range: 1 to 15, mean 7.67 foci per patient). All SRCC-pTis foci were observed in the fundic gland area and on the superficial side of the mucosa. Histologically, tumor cells of SRCC-pTis were found between normal foveolar epithelial cells and the basement membrane, following a typical pagetoid spread pattern. Immunohistochemically, E-cadherin expression was lost in SRCC-pTis (27/28, 96.4%) more frequently than in SRCC-pT1a (95/197, 48.2%; P<0.001). To elucidate the specificity of SRCC-pTis for HDGC, 60 samples (range: 0.12 to 1.49 m, total 28.8 m of mucosal length) from gastric cancer cases were analyzed as controls, in which no SRCC-pTis were identified. Our results indicate that SRCC-pTis is a distinct histologic feature with high specificity for HDGC cases with CDH1 germline mutations.