Unique irregular burrows were newly observed in the middle part of the Hasandong Formation, Galsaman Bay, Hadong County, Gyeongsangnam-do, Korea. In this study the occurrences of the burrows and burrow-bearing deposits are described and the origin of the burrows and their paleoecological and paleoenvironmental implications are discussed. The burrow-bearing deposits are floodplain deposits with calcic and vertic paleosols formed on a fluvial plain with meandering rivers. The burrows are mostly more than a few cm in diameter, and branching in L, T, and X shapes, chambers, cuspate or grooved walls, and spiraled burrows are observed, indicating that the burrows were formed by small-sized fossorial vertebrates. It is interpreted that semiarid climate with alternation of wetting and drying periods was prevailing during the formation of the burrow deposits, suggesting that the burrows were exploited for the adaptation to extreme climatic condition. The preservation of the burrows by fossorial vertebrates in the Hasandong Formation suggests that mammals and reptiles together with dinosaurs influenced the vertebrate ecosystem on the Cretaceous alluvial plain in Korean Peninsula. The study on small-sized vertebrates in paleontology, paleoecology, and paleoenvironment should be further needed to understand the Cretaceous terrestrial ecosystem in Korea.