Sexual and seasonal variation in diet quality and the gastrointestine have important implications for forage acquisition and the feeding strategy. We assessed the botanical and chemical compositions of the diet and the gastrointestinal macrostructure of the sika deer (Cervus nippon) in western Japan. The sexual dimorphism-body size hypothesis predicts that smaller females will have a better diet than larger males to meet a higher nutritional demand due to a higher metabolic rate. According to the optimal digestion theory, the gut of larger males will retain a greater quantity of digesta of poor forage to compensate for the reduction in net energy obtained per unit of digesta. The relatively greater omasum content in winter than in summer was the only feature consistent with this prediction. This may suggest that the omasum is the organ that most sensitively reflects diet quality, because the summer diet quality is little better than that in winter. Contrary to predictions, the botanical composition of the diet did not show a sexual difference, but females had greater relative weights of stomach contents, tissues, and hindgut segments (small and large intestines, and cecum-colon), and a relatively greater small intestine length than males in summer. Further, females had greater relative weights of abomasum contents, ruminoreticulum tissue, and small intestine in summer than in winter, but no seasonal variation in gastrointestinal features was detected in males. The sexual variation in diet and in the gastrointestine in summer suggest that lactating females intake more forage and maintain more digesta in the gut, due to not body size but to higher energy requirements for lactation. The greater gut fill and tissue weight of females in summer than in winter likely resulted from a nutrient demand from lactation greater than that from gestation.