Orbital metastasis may be the initial manifestation of a malignancy of unknown origin. The primary locations of orbital metastasis are usually the lung, prostate, gastrointestinal tract, skin, kidney, eye, or thyroid gland. Metastasis of gastric carcinoma to an extraocular eye muscle is extremely rare. A solitary thickening in an extraocular eye muscle with no inflammatory features is suspect for a tumor. Symptoms such as diplopia, proptosis, ptosis, vision loss, or pain may be associated with an orbital malignancy. Our patient, a 67-year-old man known with radically resected prostate cancer, presented with complaints of vertigo with a tendency to fall, headache, and diplopia when looking to the right. As a coincidental finding, swelling of the rectus lateralis muscle of the left eye was observed on imaging. Extensive additional investigations showed that a gastric carcinoma with intraorbital and leptomeningeal metastasis was the cause. In conclusion, a solitary thickened extraocular eye muscle should be recognized in time and examined further.