Helicobacter is a genus of Gram-negative bacteria that have a characteristic spiral shape and have been found in the lining of the stomach and upper gastrointestinal tract, as well as the liver of mammals and some birds. The best known species of this genus is H. pylori which is a human pathogen and is responsible for most cases of peptic ulcer and chronic gastritis; it also serves as the type species of the genus.

Helicobacter sp. are able to thrive in the strongly acidic mammalian stomach by producing large quantities of a urease, which locally raises the pH from ~2 to a more hospitable range (pH 6-7). Bacteria belonging to this genus are susceptible to antibiotics such as penicillin, are microaerophilic (require small amounts of oxygen), and are fast-moving with multiple flagella.

Initially, these bacteria were considered to be members of the genus Campylobacter, but since 1989 they are grouped in their own genus ^,.

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