A blood gas monitor is a medical device that measures the amount of a dissolved gas in a patient's blood. It is often attached to a medical monitor so staff can directly read a patient's oxygenation at all times.

By far, the most common monitor measures oxygen perfusion, although devices for measuring pO2, pCO2 (carbon dioxide) and pH values also exist. Typically it has a small light-emitting diode and photodiode on a probe clipped to a part of the patient's body. The red light reflects from the blood in a transparent part of the patient's body, such as an ear-lobe or finger-nail. As a patient's oxygenation level drops, the blood becomes more blue, reflecting less red light to the photodiode.

A blood-oxygen monitor customarily measures percent of normal. Acceptable normal ranges are from 95 to 100 percent. For a patient breathing room air, at not far above sea level, an estimate of arterial pO2 can be made from the blood-oxygen monitor SpO2 reading.

The monitor value bounces in time to the heart beat because the blood vessels expand and contract with the heartbeat. Some monitors also measure heart rate. Modern oxymeters can clip onto the finger of a patient and use optical properties of light going through a nail to determine the amounts of these chemicals. Prior to the oxymeter's invention, many complicated blood tests needed to be performed.

Blood oxygen monitors are of critical importance in emergency medicine and are also very useful for patients with respiratory or cardiac problems, as well as pilots operating in a non-pressurized aircraft above 10,000 feet (12,500 feet in the US), where supplemental oxygen is required.