noun
A person who advocates or practices humanitarianism; often used in political, social, or ethical contexts.
彼は人道主義者として世界中で活動している。
He works around the world as a humanitarian.
人道主義 is the abstract concept of humanitarianism, while 人道主義者 refers to a person who follows it.
Compound of 人道 (humanity, humaneness), 主義 (principle, -ism), and 者 (person).