A hypodermic needle (from Greek ὑπο- (hypo- = under), and δέρμα (derma = skin)), one of a category of medical tools which enter the skin, called sharps, is a very thin, hollow tube with one sharp tip. It is commonly used with a syringe, a hand-operated device with a plunger, to inject substances into the body (e.g., saline solution, solutions containing various drugs or liquid medicines) or extract fluids from the body (e.g., blood). Large-bore hypodermic intervention is especially useful in catastrophic blood loss or treating shock.

A hypodermic needle is used for rapid delivery of liquids, or when the injected substance cannot be ingested, either because it would not be absorbed (as with insulin), or because it would harm the liver. It is also useful to deliver certain medications that cannot be delivered orally due to vomiting. There are many possible routes for an injection, with intramuscular (into a muscle) and intravenous (into a vein) being the most common. A hypodermic syringe has the ability to retain liquid and blood in it up to years after the last use and a great deal of caution should be taken to use a new syringe every time.

The hypodermic needle also serves an important role in research environments where sterile conditions are required. The hypodermic needle significantly reduces contamination during inoculation of a sterile substrate. The hypodermic needle reduces contamination for two reasons: First, its surface is extremely smooth, which prevents airborne pathogens from becoming trapped between irregularities on the needle's surface, which would subsequently be transferred into the media (e.g. agar) as contaminants; second, the needle's surface is extremely sharp, which significantly reduces the diameter of the hole remaining after puncturing the membrane and consequently prevents microbes larger than this hole from contaminating the substrate.