Phlegm is a type of mucus, the sticky fluid secreted by the mucous membranes of animals. Its definition is limited to the mucus produced by the respiratory system, excluding that from the nasal passages, and particularly that which is expelled by coughing (sputum).

In medieval medicine, phlegm was counted as one of the four bodily humours, which possessed the properties of coldness and wetness and was responsible for apathetic and sluggish behaviour. This old belief is preserved in the word phlegmatic.

"Healthy" phlegm is normally clear or white. Yellow phlegm is normally a sign of an infection. Greenish or brownish phlegm is nearly always a sign of infection. Greenish or rusty phlegm or phlegm with rusty spots can also be a sign of pneumonia. Coughing up brown phlegm is also a common symptom of smoking too much. This is due to resin sticking to the viscous texture of the phlegm and being ejected by the body. Phlegm may be a carrier of larva of intestinal parasites, see hookworm.

Bloody sputum can be a symptom of serious disease (such as tuberculosis), but can also be a relatively benign symptom of a minor disease (such as bronchitis). In the latter case, the sputum is normally lightly streaked with blood. Coughing up any significant quantity of blood is always a serious medical condition and warrants immediate treatment.