A double helix is a geometrical shape consisting of two helices around the same axis. A helix is a twisting spiral shape, like a spring. The two helices in a double helix are congruent, meaning that they are the same in every dimension, though in different positions around the axis. A double helix may be either right-handed or left-handed, depending on whether it coils clockwise or counter-clockwise, respectively.

The double helix is probably best known as the shape of deoxyribonucleic acid (DNA), the material that carries genetic information in the body. The DNA double helix is right-handed and consists of two strands or backbones of phosphate and sugar joined by base pairs of the nucleotides adenine, thymine, guanine, and cytosine, often abbreviated A, T, G, and C. Adenine always joins with thymine, and guanine always joins with cytosine, so knowing the bases on one side of the double helix reveals those on the other side. These four nucleotides are responsible for encoding all of the genetic information in the human genome. The double helix shape of DNA was first published in 1953 by molecular biologists Francis Crick and James D. Watson.

The helix is a frequently occurring shape in nature, found, for example, in the vines of various plants and in many proteins in the body. Though the double helix is rarer, the shape is not confined to DNA. Another interesting example of a naturally occurring double helix is a space nebula in the Milky Way photographed in 2006.

Scientists believe that this "DNA nebula" was formed by the twisting force of magnetic field lines running through its center. The field lines may be anchored to a body orbiting the black hole at the center of the Milky Way, about 300 light years away from the nebula. Though this nebula is the only known one of its kind, researchers speculate that there are more of them throughout the universe, though possibly not within our galaxy.