The aim was to assess the effect of adherence to the Mediterranean Diet, measured with five different indexes, on the risk of gastric cancer. Data come from the multicase-control study MCC—Spain, which included 354 gastric cancer cases and 3040 controls with data on diet. We used five indexes to evaluate adherence to the Mediterranean diet and assess the association between each pattern with the risk of gastric cancer, using multivariate logistic regression. The analyses were performed for the whole set of gastric cancer cases, by anatomical location (cardia and non-cardia) and by histological type (intestinal and diffuse). According to the used index, a high adherence protects one from gastric cancer (between 48% (aOR = 0.52; CI 95% = 0.28–0.94) and 75% (aOR = 0.25; CI 95% = 0.12–0.52)), from non-cardia (between 48% (aOR = 0.52; CI 95% = 0.36–0.75) and 65% (aOR = 0.35; CI 95% = 0.23–0.52)), and from the intestinal type (between 41% (aOR = 0.59; CI 95% = 0.36–0.95) and 72% (aOR = 0.28; CI 95% = 0.16–0.50)), but not from the diffuse type. In conclusion, high adherence to a Mediterranean diet pattern is a protective factor for the risk of gastric cancer, with greater adherence leading to greater protection.