The Perseus Arm is one of two major spiral arms of the Milky Way galaxy, the second major arm being the Scutum–Centaurus Arm. The Milky Way is a barred spiral galaxy with two major arms and a number of minor arms or spurs. The Perseus Arm begins from the distal end of the long Milky Way central bar. The Perseus Arm is almost completely unexplored - a few Commonality expeditions have ventured from their colonies in the Empyrean Roots into the Near Perseus Sector in the past century, but have reported little than basic surveying and mapping information.

The Perseus Spiral Arm in the Orion Sphere area lies approximately 35,000 light-years from the galactic core, and is located between the minor Orion and Saggitarius Arms. The Perseus Arm is separated from the Orion Arm by the Great Perseus Rift, a particularly sparse region of space. The few areas of stellar density crossing the Rift tend to form thin threads that weave together connecting the two arms - the Binding Strand of the Binding Sector and the Empyrean Roots of the Auriga Sector and Perseus Rift ε Sector. It is named after the Perseus constellation, which was visible in its direction from pre-Fall Terra.