Stephenson 2-18 (St2-18, also known as Stephenson 2-18 and RSGC2-18) is a red supergiant (RSG) star. It is close to, and a possible member of, the open cluster Stephenson 2. It is about 6,000 parsecs (20,000 ly) away from Earth in the constellation of Scutum and is one of the largest known stars.