A hypodermic needle (from Greek ὑπο- (hypo- = under), and δέρμα (derma = skin)) is a very thin, hollow tube with one sharp tip. As one of the most important intravenous inventions in the field of drug administration, it is one of a category of medical tools which enter the skin, called sharps. 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. It was invented by Francis Rynd in 1844.

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.

The ancient Greeks and Romans knew injection as a method of medicinal delivery from observations of snakebites and poisoned weapons. There are also references to "anointing" and "inunction" in the Old Testament as well as the works of Homer, but injection as a legitimate medical tool was not truly explored until the 17th century.

Christopher Wren performed the earliest confirmed experiments with crude hypodermic needles, performing intravenous injection into dogs in 1656. These experiments consisted of using animal bladders (as the syringe) and goose quills (as the needle) to administer drugs such as opium intravenously to dogs. Wren and others' main interest was to learn if medicines traditionally administered orally would be effective intravenously. In the 1660s, Johann Daniel Major of Kiel and Johann Sigismund Elsholtz of Berlin were the first to experiment with injections in humans.

The 19th century saw the development of medicines that were effective in small doses, such as opiates and strychnine. This spurred a renewed interest in direct, controlled application of medicine. "Some controversy surrounds the question of priority in hypodermic medication." Irish physician Francis Rynd is generally credited with the first successful injection in 1844, in the Meath Hospital in Dublin, Ireland.

Alexander Wood's main contribution was the all-glass syringe in 1851, which allowed the user to estimate dosage based on the levels of liquid observed through the glass. Wood used hypodermic needles and syringes primarily for the application of localized, subcutaneous injection (localized anesthesia) and therefore was not as interested in precise dosages.

Simultaneous to Wood's work in Edinburgh, Charles Pravaz of Lyon also experimented with sub-dermal injections in sheep using a syringe of his own design. Pravaz designed a hypodermic needle measuring 3 cm (1.18 in) long and 5 mm (0.2 in) in diameter; it was made entirely of silver.^{[citation needed]}