Acid-fast bacteria (also known as acid-fast bacilli or AFB) are microorganisms resistant to acids and/or ethanol-based decolorization procedures common in many staining protocols, hence, the term acid-fast.

One notable example of acid-fast bacteria is mycobacteria, including Mycobacterium tuberculosis who causes the respiratory disease tuberculosis and Mycobacterium leprae that causes the disfiguring disease leprosy. Very few structures are acid-fast, which makes identifying acid-fastness particularly useful in diagnosis.