A solitary extraosseous plasmacytoma is a rare type of plasma cell neoplasm. Its occurrence in the stomach is particularly unusual and can easily be mistaken for more common types of tumors. We describe a case involving a solitary extraosseous plasmacytoma in a patient who experienced weight loss as the sole symptom. Initially, the condition was misdiagnosed as gastric adenocarcinoma based on endoscopic biopsy results, leading to a gastrectomy after neoadjuvant chemotherapy. Subsequent examination of the pathological specimen revealed the presence of plasma cell neoplasia alongside a gastric adenocarcinoma in situ.