18 Delphini b, also known as Arion, is an extrasolar planet located 238 light years away from Earth in the constellation Delphinus.

The 993-day period planet orbits the yellow giant star 18 Delphini. It is a very massive and dense planet with a minimum mass of 10.3 MJ it was discovered on February 19, 2008 by Sato,

In July 2014, the International Astromonical Union launched a process for giving proper names to certain exoplanets and their host stars. The process involved public nomination and voting for the new names. In December 2015, the IAU announced the name "Arion" for this planet. The name was submitted by the Tokushima Prefectural Jonan High School Science Club of Japan. Arion was a genius of poetry and music in ancient Greece. According to legend, his life was saved at sea by dolphins after attracting their attention by the playing of his kithara.