A lumbar puncture is a procedure in which a needle is inserted into the spinal canal to measure the pressure and obtain a sample of cerebrospinal fluid (CSF), the colourless fluid that surrounds the brain and spinal cord. The procedure may also be used to inject anesthetics, medicine, or a contrast dye (for spinal X-rays) into the spinal fluid or to drain fluids that accumulate as a result of certain medical conditions.

Fluid samples obtained from a lumbar puncture can be analyzed for signs of infection (such as meningitis), inflammation, cancer, or bleeding in the area around the brain or spinal cord.

A lumbar puncture is also sometimes called a spinal tap.