Colorectal cancer is considered the third most common cancer worldwide. On the other hand, gallbladder cancer is rare. Synchronous tumors in both the colon and the gallbladder are extremely infrequent. Herein, we report the case of a female patient with sigmoid colon cancer and incidental detection of synchronous gallbladder cancer on histopathological examination of the surgical specimen. As synchronous gallbladder and colonic carcinomas are rare, physicians should be aware of these so that an optimal course of treatment can be chosen.