Walter Thomas Mills (1856–1942) was an American socialist activist, educator, lecturer, writer, and newspaper publisher. He is best remembered for the role he played in the Socialist Party of America during the first decade of the 20th Century as one of the leaders of the organization's moderate wing. He also was a key actor in the labor movement of New Zealand as a founder of the United Labour Party in 1912. He returned to the United States in 1914 with the advent of World War I and worked unsuccessfully to keep the country out of the bloody European conflict, eventually leaving the socialist movement in the 1920s.