Stomatitis is inflammation of the mouth and lips. It refers to any inflammatory process affecting the mucous membranes of the mouth and lips, with or without oral ulceration.

In its widest meaning, stomatitis can have a multitude of different causes and appearances. Common causes include infections, nutritional deficiencies, allergic reactions, radiotherapy, and many others.

When inflammation of the gums and the mouth generally presents itself, sometimes the term gingivostomatitis is used, though this is also sometimes used as a synonym for herpetic gingivostomatitis.

The term is derived from the Greek stoma (στόμα), meaning "mouth", and the suffix -itis (-ῖτις), meaning "inflammation".