Travelers' diarrhea (TD) is a stomach and intestinal infection. TD is defined as the passage of unformed stool (one or more by some definitions, three or more by others) while traveling. It may be accompanied by abdominal cramps, nausea, fever, headache and bloating. Occasionally bloody diarrhea may occur. Most travelers recover within three to four days with little or no treatment. About 10% of people may have symptoms for a week.

Bacteria are responsible for more than half of cases, typically via foodborne illness and waterborne diseases. The bacteria enterotoxigenic Escherichia coli (ETEC) are typically the most common except in Southeast Asia, where Campylobacter is more prominent. About 10 to 20 percent of cases are due to norovirus. Protozoa such as Giardia may cause longer term disease. The risk is greatest in the first two weeks of travel and among young adults. People affected are more often from the developed world.

Recommendations for prevention include eating only properly cleaned and cooked food, drinking bottled water, and frequent hand washing. The oral cholera vaccine, while effective for cholera, is of questionable use for travelers' diarrhea. Preventive antibiotics are generally discouraged. Primary treatment includes rehydration and replacing lost salts (oral rehydration therapy). Antibiotics are recommended for significant or persistent symptoms, and can be taken with loperamide to decrease diarrhea. Hospitalization is required in less than 3 percent of cases.

Estimates of the percentage of people affected range from 20 to 50 percent among travelers to the developing world. TD is particularly common among people traveling to Asia (except for Japan and Singapore), the Middle East, Africa, Latin America, and Central and South America. The risk is moderate in Southern Europe, Russia, and China. TD has been linked to later irritable bowel syndrome and Guillain–Barré syndrome. It has colloquially been known by a number of names, including "Montezuma's revenge," the "Nile runs" and "Delhi belly".