S100 protein family is small calcium-binding proteins with two EF-hand motifs and comprises more than 20 proteins in human. Although S100A proteins are known to play important roles in proinflammatory responses including damage-associated molecular pattern (DAMP) signaling and in the establishment of pregnancy, the expression of S100As have not been determined in the uterine endometrium during the estrous cycle in pigs. Thus, this study was performed to investigate expression and localization of S100A8, S100A9, and S100A12 in the uterine endometrial tissues during the estrous cycle in pigs. Real-time RT-PCR analysis showed that S100A8, S100A9, and S100A12 mRNAs were expressed in the uterine endometrium during the estrous cycle with higher levels on days 15 and 18 of the estrous cycle than the other days of cycle. Immunohistochemistry analysis showed that S100A9 and S100A12 proteins were mainly localized to the immune cells in the uterine endometrium. Especially, S100A9- and S100A12-positive immune cells were detected in the uterine blood vessels on day 15 of the estrous cycle, and also localized to stroma near to luminal epithelium on days 0 and 18 of the estrous cycle. These results showed that S100As were expressed in the uterine endometrium during the estrous cycle in a cyclic stage-specific manner, and these proteins were localized to the immune cells in the endometrium. These suggest that immune cells expressing S100A proteins may be recruited into the endometrium during the estrous cycle and play an important role in regulating endometrial function in pigs.