Spiral arms are a defining feature of spiral galaxies. They manifest as spiral-shaped regions of enhanced brightness within the galactic disc. Typically, spiral galaxies exhibit two or more spiral arms. The collective configuration of these arms is referred to as the spiral pattern or spiral structure of the galaxy.

The appearance of spiral arms is quite diverse. Grand design spiral galaxies exhibit a symmetrical and distinct pattern, comprising two spiral arms that extend throughout the galaxy. In contrast, the spiral structure of flocculent galaxies comprises numerous small fragments of arms that are not connected to each other. The appearance of spiral arms varies across the electromagnetic spectrum.

In addition to increased brightness, spiral arms are characterised by an increased concentration of interstellar gas and dust, bright stars and star clusters, active starburst, a bluer colour, and an enhanced magnetic field strength in galaxies. The contribution of spiral arms to the total galaxy luminosity can reach 40–50% for some galaxies. The characteristics of spiral arms are correlated with other properties of galaxies, for example, the twist angle of spiral arms is related to parameters such as the mass of the supermassive black hole at the centre and the contribution of the bulge to the total luminosity.

Two main theories have been proposed to explain the origin of spiral arms: the stochastic self-propagating star formation model and the density wave theory. These theories describe different variants of the spiral structure and do not exclude each other. In addition to these theories, there are other theories that can explain the appearance of spiral structure in some cases.

The spiral structure was first identified in 1850 by Lord Rosse in the galaxy M51. The nature of spiral structure in galaxies remained unresolved for a considerable period of time.

Spiral arms are a defining feature of the structural composition of spiral galaxies, which are situated within discs and exhibit heightened brightness relative to their surrounding environment. Such structures take the form of spirals, which in unbarred galaxies usually originate from a region near the centre of the galaxy, whereas in barred galaxies they originate at the ends of the bar. The spiral arms do not extend over the entire radius of the disc and cease at the distance at which the disc can still be discerned. A galaxy typically comprises two or more spiral arms. The collective configuration of these arms within a galaxy is referred to as a spiral pattern or spiral structure.

Around two thirds of all massive galaxies are spiral galaxies. Spiral arms have been observed in galaxies at redshifts up to ${\displaystyle z\approx 1}$, and on occasion even at greater distances, which corresponds to a time when the age of the Universe was less than half of the present one. This suggests that the spiral structure is a long-lived phenomenon.

The spiral arms exhibit considerable variation in their appearance. In general, they are characterized by an increased concentration of gas and dust, active starburst, and a greater prevalence of star clusters, H II regions, and bright stars than in the remainder of the disk. While spiral arms are primarily identifiable due to their young stellar population, there also exists an increased concentration of old stars within them.