Hypercapnia or hypercapnea (from the Greek hyper = "above" and kapnos = "smoke"), also known as hypercarbia, is a condition where there is too much carbon dioxide (CO₂) in the blood. Carbon dioxide is a gaseous product of the body's metabolism and is normally expelled through the lungs.

Hypercapnia normally triggers a reflex which increases breathing and access to oxygen, such as arousal and turning the head during sleep. A failure of this reflex can be fatal, as in sudden infant death syndrome.

Hypercapnia is the opposite of hypocapnia.

Hypercapnia is generally caused by hypoventilation, lung disease, or diminished consciousness. It may also be caused by exposure to environments containing abnormally high concentrations of carbon dioxide (usually due to volcanic or geothermal causes), or by rebreathing exhaled carbon dioxide. It can also be an initial effect of administering supplemental oxygen on a patient with sleep apnea. In this situation the hypercapnia can also be accompanied by respiratory acidosis.