BACKGROUND Low-grade appendiceal mucinous neoplasms (LAMNs) are cytologically low-grade tumors of the appendix and are a frequent cause of pseudomyxoma peritonei. They can become a diagnostic challenge when they metastasize to the ovaries, where they may mimic primary ovarian mucinous tumors. CASE REPORT We report the case of a patient with very large bilateral ovarian mucinous tumors and a concurrent minute LAMN incidentally discovered in a grossly normal appendix. A primary ovarian tumor was suspected, but histological analysis of the ovaries suggested an appendiceal origin. Immunohistochemical studies were not informative and a consensus regarding the source of the ovarian tumors could not be reached within our department. Subsequent next-generation sequencing of tumors from the right ovary, left ovary, appendix, and matched normal tissue demonstrated identical somatic point mutations in KRAS and GNAS present in all tumors. The patient was diagnosed with metastatic LAMN and did not receive further treatment. She remains disease-free after 15 months of close observation. CONCLUSIONS Determining the tissue of origin in low-grade mucinous tumors of the ovaries can be challenging when a concurrent LAMN is identified in the appendix. In cases where histology and immunohistochemistry are insufficient to render a diagnosis, the presence of concurrent KRAS and GNAS mutations in both tumors strongly favors a diagnosis of metastatic LAMN. We emphasize the utility of targeted next-generation sequencing to establish tissue of origin in challenging cases when LAMN is suspected as the source of mucinous ovarian tumors.