BACKGROUND: The role of janus kinase (JAK)/signal transducer and activator of transcription (STAT) signaling remains unclear in gastric solid-type poorly differentiated adenocarcinoma. The present study investigates the clinicopathological significance of JAK2, STAT3, and STAT4 expression in solid-type poorly differentiated adenocarcinoma. METHODS: We retrospectively enrolled 102 participants with primary solid-type poorly differentiated adenocarcinoma. We categorized participants according to deficient or proficient mismatch repair status (46 and 56 participants, respectively). Expression of phosphorylated JAK2 (pJAK2), phosphorylated STAT3 (pSTAT3), and STAT4 were analyzed via immunohistochemistry. We analyzed differences in protein expression in relation to mismatch repair status, and associations of high/low protein expression with clinicopathological characteristics and prognoses. RESULTS: Deficient mismatch repair was found to be associated with high pJAK2 (p = 0.038) and STAT4 (p = 0.023) expression in contrast to proficient mismatch repair. Log-rank analysis revealed high pSTAT3 and low STAT4 expression to be significantly correlated with reduced overall survival (p = 0.001). Multivariate analysis revealed high pSTAT3 and low STAT4 expression to be independent indicators of unfavorable prognosis (hazard ratio = 2.751, p = 0.030), as was proficient mismatch repair status (hazard ratio = 3.819, p = 0.012). CONCLUSIONS: High expression of pJAK2 and STAT4 is more frequent in deficient compared with proficient mismatch repair in solid-type poorly differentiated adenocarcinoma. High pSTAT3 and low STAT4 expression could be a useful prognostic indicator in solid-type poorly differentiated adenocarcinoma.