A pseudoaneurysm, also known as a false aneurysm, is a locally-contained hematoma outside an artery or the heart due to damage to the vessel wall. The injury passes through all three layers of the arterial wall, causing a leak, which is contained by a new, weak, "wall" formed by the products of the clotting cascade. A pseudoaneurysm does not contain any layer of the vessel wall.

This differentiates it from an true aneurysm, which has all the three layers of the vessel wall, and a dissecting aneurysm, where there is a breach in the innermost layer of an artery and subsequent dissection/separation between the tunica intima and the tunica media.

Pseudoaneurysm, being close to the vessel, can be pulsatile; it may be mistaken for a true aneurysm or dissecting aneurysm, or vice versa.

The most common presentation of pseudoaneurysm is femoral artery pseudoaneurysm following access for an endovascular procedure, and this event may complicate up to 8% of vascular interventional procedures. Small pseudoaneurysms are capable of spontaneous clotting, while others need definitive treatment.

A pseudoaneurysm may also occur in a chamber of the heart following myocardial damage due to ischemia or trauma. A pseudoaneurysm of the left ventricle is a potentially lethal complication of a heart attack.