noun
conspiracy theorist
Refers to a person who believes in or promotes conspiracy theories.
彼は有名な陰謀論者だ。
He is a well-known conspiracy theorist.
陰謀論 is the conspiracy theory itself, while 陰謀論者 is the person who believes in it.
Compound of 陰謀 (conspiracy) + 論 (theory) + 者 (person).