A pulse oximeter is a medical device that indirectly measures the amount of oxygen in a patient's blood (as opposed to measuring oxygen saturation directly through a blood sample) and changes in blood volume in the skin, producing a photoplethysmograph. It is often attached to a medical monitor so staff can see a patient's oxygenation at all times. Most monitors also display the heart rate.

A blood-oxygen monitor displays the percentage of arterial hemoglobin in the oxyhemoglobin configuration. 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.

A pulse oximeter  is a particularly convenient non-invasive measurement instrument. Typically it has a pair of small light-emitting diodes (LEDs) facing a photodiode through a translucent part of the patient's body, usually a fingertip or an earlobe. One LED is red, with wavelength of 660 nm, and the other is infrared, 905, 910, or 940 nm. Absorption at these wavelengths differs significantly between oxyhemoglobin and its deoxygenated form, therefore from the ratio of the absorption of the red and infrared light the oxy/deoxyhemoglobin ratio can be calculated. The absorbance of oxyhemoglobin and deoxyhemoglobin is the same (isosbestic point) for the wavelengths of 590 and 805 nm; earlier oximeters used these wavelengths for correction for hemoglobinconcentration.

The monitored signal bounces in time with the heart beat because the arterial blood vessels expand and contract with each heartbeat. By examining only the varying part of the absorption spectrum (essentially, subtracting minimum absorption from peak absorption), a monitor can ignore other tissues or nail polish and discern only the absorption caused by arterial blood. Thus, detecting a pulse is essential to the operation of a pulse oximeter and it will not function if there is none.

Because of their simplicity and speed (they clip onto a finger and display results within a few seconds), pulse oximeters 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. Prior to the oximeter's invention, many complicated blood tests needed to be performed.

The latest generation pulse oximeter use digital signal processing to make accurate measurements in clinical conditions that were otherwise impossible. These include situations of patient motion, low perfusion, bright ambient light, and electrical interference. Because of their insensitivity to non-pulsate signals, it is also possible to build reflectance probes that place the photodiode beside the LEDs and can be placed on any flat tissue. These can be used on non-translucent body parts, to measure pulses in specific body parts (useful in plastic surgery), or when more convenient sites are unavailable (severe burn victims). They are commonly applied to the forehead of patients with poor peripheral perfusion.

Oximetry is not a complete measure of respiratory sufficiency. A patient suffering from hypoventilation (poor gas exchange in the lungs) given 100% oxygen can have excellent blood oxygen levels while still suffering from respiratory acidosis due to excessive carbon dioxide.

It is also not a complete measure of circulatory sufficiency. If there is insufficient bloodflow or insufficient hemoglobin in the blood (anemia), tissues can suffer hypoxia despite high oxygen saturation in the blood that does arrive.

A higher level of methemoglobin will tend to cause a pulse oximeter to read closer to 85% regardless of the true level of oxygen saturation. It also should be noted that the inability of two-wavelength saturation level measurement devices to distinguish carboxyhemoglobin due to carbon monoxide inhalation from oxyhemoglobin must be taken into account when diagnosing a patient in emergency rescue, e.g., from a fire in an apartment. A CO-oximeter measures absorption at additional wavelengths to distinguish CO from O2 and determines the blood oxygen saturation more reliably. In 2005 Masimo Corporation introduced the first FDA-approved pulse oximeter to monitor carbon monoxide levels noninvasively.