noun
Refers to a person who advocates or supports communism. Neutral term in political contexts.
彼は共産主義者として知られている。
He is known as a communist.
共産主義 is the ideology (communism), while 共産主義者 is a person who follows it.
Compound of 共産 (kyōsan, 'communist production') + 主義 (shugi, 'principle, -ism') + 者 (sha, 'person'). The term is a calque from Western political vocabulary.