The patellar reflex is a type of deep tendon reflex which occurs when an area just below the patella, also known as the kneecap, is struck. In healthy individuals, when the right spot is tapped, this causes the lower leg to kick out almost instantaneously. Doctors may check the patellar reflex during a routine neurological exam, looking for reflexes which are exaggerated, delayed, or not present.

This reflex is what is known as a monosynaptic reflex, because only one synapse needs to be crossed to complete the circuit which triggers the reflex. When the area below the kneecap is hit with a reflex hammer, it hits the patellar tendon, which causes the quadriceps muscle in the thigh to contract, leading the leg to kick out. This involuntary response does not involve the brain, only the spinal cord, and while it feels instantaneous to the observer, around 50 milliseconds are actually involved in the response time, as people would see if they saw a radically slowed film of the event.

If someone does not have a patellar reflex, he or she is said to be exhibiting Westphal's sign. This indicates that there is a problem in the patient's spinal cord or peripheral nerves. The doctor usually opts assess the reflex on both legs to see the extent of the problem. It is also possible for a patient to experience an exaggerated patellar reflex, in which the leg kicks out more radically than would be expected.

A number of reflexes can be used to assess physical and neurological health. Patellar reflexes provide information about specific nerves in the leg involved, along with the spinal cord, and they may be used in routine physicals to check on a patient's health, as well as in specific neurological exams to explore possible causes for neurological symptoms. If the patellar reflex is abnormal, a doctor may recommend additional testing to learn more about the cause of the abnormality, and to start developing a diagnosis, along with treatment options.

The patellar reflex is so well-known that the common name for it, “kneejerk reflex,” is sometimes used to describe a situation in which someone responds to something without really thinking. In a metaphorical kneejerk reflex, someone can lash out verbally instead of kicking physically, sometimes causing social tension. This reflex can also be observed in animals, and it is used in routine neurological screening by veterinarians as well as doctors.