Gallbladder carcinosarcoma with osteoclast-like multinucleated giant cells is known to be most uncommon form of gallbladder cancer. Owing to its rarity, the pathogenesis of gallbladder carcinosarcoma with osteoclast-like multinucleated giant cells is largely unknown. We present a case of carcinosarcoma with osteoclast-like multinucleated giant cells in the gallbladder. A 57-year-old woman visited our hospital due to jaundice. An examination revealed calculous cholecystitis and gallbladder carcinoma. After cholecystectomy, macroscopic examination disclosed one whitish mass and another distinct brown and pendulous mass in the body of the gallbladder. A pathological examination revealed that each mass had a different histological type: adenosquamous carcinoma and carcinosarcoma with osteoclast-like multinucleated giant cells. Immunohistochemistry revealed that these osteoclast-like multinucleated giant cells are CD68(+), CD163(-), and MIB-1(-). In addition, the osteoclast-like multinucleated giant cells showed the strong expression of RANK and sarcoma cells around the osteoclast-like multinucleated giant cells, were positive for RANKL. Furthermore, RUNX2 was positive for some sarcoma cells. The result indicated that osteoclastic and osteoblast-like differentiation occurred in our case. To our knowledge, this is the first case to show the interaction of RANK-RANKL signaling in gallbladder carcinosarcoma with osteoclast-like multinucleated giant cells.