An extremely rare case of mixed adenoneuroendocrine carcinoma (MANEC) of the gallbladder is reported, in which the tumor showed diverse differentiation toward neuroendocrine tumor (grade 2), tubular adenocarcinoma, squamous cell carcinoma, and, in addition, osteosarcoma. The patient was an 80-year-old man, who underwent cholecystectomy because of a large polypoid tumor filling the gallbladder lumen. The tumor consisted of an intimate admixture of neuroendocrine tumor (grade 2) and tubular adenocarcinoma, and, in many areas, cell nests of neuroendocrine tumor appeared to bud off from tubular structures of the adenocarcinoma, in a manner resembling the ‘ductulo-insular complex’ seen in nesidioblastosis of the pancreas. Small areas of squamous cell carcinoma were also found. The stroma consisted of a dense proliferation of atypical spindle cells showing focal osteosarcomatous differentiation, and an apparent transition from an epithelial tissue component to atypical spindle cells was observed. Immunohistochemically, neuroendocrine cells were positive for synaptophysin, chromogranin A, and serotonin, and the sarcomatoid stroma was partly immunoreactive for cytokeratin. The present case is the first example of MANEC of the gallbladder with simultaneous squamous cell carcinomatous and osteosarcomatous differentiation. ‘Neometaplasia’ of carcinoma cells in diverse directions was considered the most plausible explanation for the formation of this multifaceted neoplasm.