Pneumocystis pneumonia (PCP) is a form of pneumonia, caused by the yeast-like fungus Pneumocystis jirovecii.

Pneumocystis pneumonia is not commonly found in the lungs of healthy people, but, being a source of opportunistic infection, it can cause a lung infection in people with a weak immune system. Pneumocystis pneumonia is especially seen in people with cancer undergoing chemotherapy, HIV/AIDS, and the use of medications that suppress the immune system.