18 Delphini b (abbreviated 18 Del b), formally named Arion /əˈraɪən/, is an extrasolar planet approximately 249 light-years away in the constellation of Delphinus.

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

In July 2014, the International Astronomical Union launched NameExoWorlds, 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 winning 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 ('Delphinus' is Latin for 'dolphin').

• 41 Lyncis b
• Xi Aquilae b

• Jean Schneider (2011). "Notes for Planet 18 Del b". Extrasolar Planets Encyclopaedia. Retrieved 30 September 2011.

• da Silva, L.; et al. (November 2006), "Basic physical parameters of a selected sample of evolved stars", Astronomy and Astrophysics, 458 (2): 609–623, arXiv:astro-ph/0608160, Bibcode:2006A&A...458..609D, doi:10.1051/0004-6361:20065105, S2CID 9341088 – lists data about the star.