A precordial thump is a medical procedure used in the initial response to a witnessed cardiac arrest when no defibrillator is immediately available. It can be used as a small part of the provision of advanced cardiac life support (ACLS).

To perform a precordial thump, a highly trained provider such as a paramedic or physician strikes a single very carefully aimed blow with the fist to the center of the patient's sternum. The intent is to possibly interrupt a heart-damaging rhythm.

While the odds of success are poor, if a precordial thump is done properly little time is lost. The provider will immediately continue with other ACLS skills, or CPR if the needed ACLS drugs and equipment (such as a defibrillator) are unavailable.

The performance of a precordial thump is outside the scope of first aid and requires at minimum training in advanced cardiac life support. ACLS is performed primarily by physicians, paramedics and nurses with advanced training in emergency care.

The use of the precordial thump technique has sometimes been shown in movies and television, usually in passing without any explanation. Untrained laypersons have been known to attempt it and sometimes cause additional injury to the patient as the blow must be carefully aimed.