noun
imperialist
Refers to a person who supports or advocates imperialism. Often used in political or historical contexts.
彼は帝国主義者として批判された。
He was criticized as an imperialist.
帝国主義 is the ideology of imperialism, while 帝国主義者 is a person who supports it.
Compound of 帝国 (ていこく, 'empire') + 主義 (しゅぎ, '-ism') + 者 (しゃ, 'person').