A knitting needle or knitting pin is a tool in hand-knitting to produce knitted fabrics. They generally have a long shaft and taper at their end, but they are not nearly as sharp as sewing needles. Their purpose is two-fold. The long shaft holds the active (unsecured) stitches of the fabric, to prevent them from unravelling, whereas the tapered ends are use to form new stitches. Most commonly, a new stitch is formed by inserting the tapered end through an active stitch, catching a loop (also called a bight) of fresh yarn and drawing it through the stitch; this secures the initial stitch and forms a new active stitch in its place. In specialized forms of knitting, however, the needle may be passed between active stitches being held on another needle, or indeed between/through inactive stitches that have been knit previously.

The size of a needle is described first by its radius, and secondarily by its length. The size of the new stitch is determined in large part by the radius of the knitting needle used to form it, because that affects the length of the yarn-loop drawn through the previous stitch. Thus, large stitches can be made with large needles, whereas fine knitting requires fine needles. In most cases, the knitting needles being used in hand-knitting are of the same radius; however, in uneven knitting, needles of different sizes may be used. A similar effect on stitch size can be obtained, however, by wrapping the yarn multiple times about a single needle. The length of a needle determines how many stitches it can hold at once; for example, very large projects such as a shawl with hundreds of stitches might require a long needle. Various sizing systems for needles are in common use.

Knitting needles come in various types. Perhaps the most common is a pair of long, straight, rigid needles that are capped at one end. A circular needle is another type, in which two tapered rigid ends are connected by a long, flexible cord; the tapered ends are used for creating new stitches, whereas the flexible cord holds the active stitches. A third type is a shorter, straight, rigid needle that is tapered at both ends; such double-pointed needles are usually used in sets of four or five, and most often for circular knitting in which the radius is very small, e.g., sweater sleeves or socks. A fourth type of knitting needle is the very short, double-pointed cable needle, which is used to hold stitches while the order of stitches being knit is permuted in cable knitting.

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