Whether the positive associations of gastric cancer (GC) with autoimmune diseases are causal has always been controversial. This study aims to estimate the causal relationship between GC and 12 autoimmune diseases by means of Mendelian randomization (MR) analysis.After rigorous evaluation, potential candidate single nucleotide polymorphisms (SNPs) for GC and 12 autoimmune diseases were extracted from genome-wide association study (GWAS) datasets. We performed the MR analyses using the inverse variance weighted (IVW) method as the primary approach to the analysis. Three sensitivity analysis methods were added to assess the robustness of the results. In addition, heterogeneity was measured using Cochran's Q-value, and horizontal pleiotropy was assessed using MR-Egger regression and leave-one-out analysis.The IVW result, which is the main method of analysis, shows no evidence of a causal association between GC and any autoimmune disease. The results of IVW analysis show the relationship between rheumatoid arthritis (p = 0.1389), systemic lupus erythematosus (p = 0.1122), Crohn's disease (p = 0.1509), multiple sclerosis (p = 0.3944), primary sclerosing cholangitis (p = 0.9022), primary biliary cirrhosis (p = 0.7776), type 1 diabetes (p = 0.9595), ulcerative colitis (p = 0.5470), eczema (p = 0.3378), asthma (p = 0.7436), celiac disease (p = 0.4032), and psoriasis (p = 0.7622) and GC susceptibility. The same result was obtained with the weighted median and the MR-egger (p > 0.05).Our study did not find a genetic causal relationship between susceptibility to these autoimmune diseases and GC, which suggests that unmeasured confounders (e.g., inflammatory processes) or shared genetic architecture may be responsible for the reported epidemiologic associations. Further studies of ancestral diversity are warranted to validate such causal associations.