Therion is a Swedish symphonic metal band founded by Christofer Johnsson in 1987. The word "therion" comes from the Greek therion (θηρίον), meaning "Beast," i.e., that of the Christian Book of Revelation. However, band's name is also taken as homage to the Celtic Frost album To Mega Therion.

Beginning as a death metal band, they later turned to combining orchestral elements with their metal music, employing heavy use of choirs and classical musicians, not only as additions to but also as integral parts of the composition. The band is one of the first both commercial and audience successful heavy metal with fully live orchestra featured. It is also one of the bands which originated, popularised and highly influenced the symphonic metal genre, cited as "the most adventurous metal band at present".

Therion's music takes its themes from different mythologies and is based on concepts ranging from occultism, magic and ancient traditions and writings. The majority of their lyrics are written by Thomas Karlsson, head and founder of the magical order Dragon Rouge, of which Johnsson is a member.The band has seen many changes in line-up and style throughout its history.