noun
Refers to a person who adheres to naturalism in philosophy, literature, or art. Not typically used for a field biologist; for that, 博物学者 (はくぶつがくしゃ) is more common.
彼は文学における自然主義者として知られている。
He is known as a naturalist in literature.
博物学者 is the common term for a naturalist in the sense of a field biologist or a person who studies natural history, while 自然主義者 is used for a proponent of naturalism as a philosophical or artistic movement.
Compound of 自然主義 (しぜんしゅぎ, 'naturalism') + 者 (しゃ, 'person'). The term is a direct calque from Western philosophical vocabulary.