Australian physician who won, with J. Robin Warren (Warren, J. Robin), the 2005 Nobel Prize for Physiology or Medicine for their discovery that stomach ulcers (peptic ulcer) are an infectious disease caused by bacteria.

Marshall obtained a bachelor's degree from the University of Western Australia in 1974. From 1977 to 1984 he worked at Royal Perth Hospital, and he later taught medicine at the University of Western Australia, where he also was a research fellow.

In the early 1980s Marshall became interested in the research of Warren, who in 1979 had first observed the presence of spiral-shaped bacteria in a biopsy of a patient's stomach lining. The two began working together to determine the significance of the bacteria. They conducted a study of stomach biopsies from 100 patients that systematically showed that the bacteria were present in almost all patients with gastritis, duodenal ulcer, or gastric ulcer. Based on these findings, Warren and Marshall proposed that the bacterium Helicobacter pylori was involved in causing those diseases. This contradicted the commonly held belief that peptic ulcers resulted from an excess of gastric acid that was released in the stomach as the result of emotional stress, the ingestion of spicy foods, or other factors. It also challenged the traditional treatments, which included antacid medicines and dietary changes, by supporting a curative regimen of antibiotics and acid-secretion inhibitors. Hoping to persuade skeptics, Marshall drank a culture of H. pylori and within a week began suffering stomach pain and other symptoms of acute gastritis. Stomach biopsies confirmed that he had gastritis and showed that the affected areas of his stomach were infected with H. pylori. Marshall subsequently took antibiotics and was cured.

Prior to winning the Nobel Prize, Marshall had received the Albert Lasker Clinical Medical Research Award (1995) and the Benjamin Franklin Medal (1999) for his work on H. pylori. He also wrote several books, including Helicobacter Pioneers (2002), a collection of historical first-hand accounts of scientists who studied Helicobacter.