Stomach cancer, also known as gastric cancer, is a malignant tumor of the stomach. It is a cancer that develops in the lining of the stomach, caused by abnormal cell growth. Most cases of stomach cancers are gastric carcinomas, which can be divided into several subtypes, including gastric adenocarcinomas. Lymphomas and mesenchymal tumors may also develop in the stomach. Early symptoms may include heartburn, upper abdominal pain, nausea, and loss of appetite. Later signs and symptoms may include weight loss, yellowing of the skin and whites of the eyes, vomiting, difficulty swallowing, and blood in the stool, among others. The cancer may spread from the stomach to other parts of the body, particularly the liver, lungs, bones, lining of the abdomen, and lymph nodes.

The bacterium Helicobacter pylori accounts for more than 60% of cases of stomach cancer. Certain strains of H. pylori have greater risks than others. Smoking, dietary factors such as pickled vegetables and obesity are other risk factors. About 10% of cases run in families, and between 1% and 3% of cases are due to genetic syndromes inherited such as hereditary diffuse gastric cancer. Most of the time, stomach cancer develops in stages over the years. Diagnosis is usually by biopsy done during endoscopy. This is followed by medical imaging to determine if the cancer has spread to other parts of the body. Japan and South Korea, two countries that have high rates of the disease, screen for stomach cancer.

A Mediterranean diet lowers the risk of stomach cancer, as does not smoking. Tentative evidence indicates that treating H. pylori decreases the future risk. If stomach cancer is treated early, it can be cured. Treatments may include some combination of surgery, chemotherapy, radiation therapy, and targeted therapy. For certain subtypes of gastric cancer, cancer immunotherapy is an option as well. If treated late, palliative care may be advised. Some types of lymphoma can be cured by eliminating H. pylori. Outcomes are often poor, with a less than 10% five-year survival rate in the Western world for advanced cases. This is largely because most people with the condition present with advanced disease. In the United States, five-year survival is 31.5%, while in South Korea it is over 65% and Japan over 70%, partly due to screening efforts.

Globally, stomach cancer is the fifth-leading type of cancer and the third-leading cause of death from cancer, making up 7% of cases and 9% of deaths. In 2018, it newly occurred in 1.03 million people and caused 783,000 deaths. Before the 1930s, it was a leading cause of cancer deaths in the Western world; rates have sharply declined among younger generations in the West, although they remain high for people living in East Asia. The decline in the West is believed to be due to the decline of salted and pickled food consumption, as a result of the development of refrigeration as a method of preserving food. Stomach cancer occurs most commonly in East Asia, followed by Eastern Europe. It occurs twice as often in males as in females.

Stomach cancer is often either asymptomatic (producing no noticeable symptoms) or it may cause only nonspecific symptoms (which may also be present in other related or unrelated disorders) in its early stages. By the time symptoms are recognized, the cancer has often reached an advanced stage (see below) and may have metastasized (spread to other, perhaps distant, parts of the body), which is one of the main reasons for its relatively poor prognosis. Stomach cancer can cause the following signs and symptoms: Unexplained nausea, vomiting, diarrhoea, and constipation. Patients can also experience unexplained weight loss.

Early cancers may be associated with indigestion or a burning sensation (heartburn). However, fewer than one in every 50 people referred for endoscopy due to indigestion has cancer. Abdominal discomfort and loss of appetite can occur.

Gastric cancers that have enlarged and invaded normal tissue can cause weakness, fatigue, bloating of the stomach after meals, abdominal pain in the upper abdomen, nausea and occasional vomiting. Further enlargement may cause weight loss or bleeding with vomiting blood or having blood in the stool, the latter apparent as black discolouration (melena) and sometimes leading to anemia. Dysphagia suggests a tumour in the cardia or extension of the gastric tumour into the esophagus.^{[citation needed]}

These can be symptoms of other problems such as a stomach virus, gastric ulcer, or tropical sprue.^{[citation needed]}