Collision tumors of the stomach are uncommon. To the best of our knowledge, this is the first case report of gastric collision tumor composed of gastrointestinal stromal tumor (GIST) intermixed with primary adenocarcinoma in the English literature. The adenocarcinoma was determined to be the primary tumor based on histologic features. The tumor cells of the GIST were diffusely and strongly positive for CD34 and CD117, weakly positive for smooth muscle actin (5% of cells), and negative for desmin, S-100 protein, synaptophysin, and cytokeratin. There was no transition between the different components. We hypothesized that the stomach was influenced by the same unknown carcinogen, resulting in a simultaneous proliferation of different cell lines (epithelial and stromal cell). This case represents an example of two independent tumors in a unique one-on-another pattern, namely growth of adenocarcinoma on GIST.