noun
Refers to a person who advocates or supports nationalism, often in a political context.
彼は熱心な国家主義者だ。
He is an ardent nationalist.
国家主義 is the ideology of nationalism, while 国家主義者 refers to a person who holds that ideology.
Compound of 国家 (nation) + 主義 (doctrine, -ism) + 者 (person).