conspiracy theorist UK /kənˈspɪɹ.ə.si ˌθɪəɹɪst/US /kənˈspɪɹ.ə.si ˌθi.ɚ.əst/ noun1 •One who believes in, follows, or advances a conspiracy theory.Mike's a conspiracy theorist; he thinks NASA faked the moon landings. formsconspiracy theorists