Clarence Ollson Senior (1903–1974) was an American socialist political activist best remembered as the National Executive Secretary of the Socialist Party of America during the 1930s. Originally a protégé of Presidential candidate Norman Thomas, during the inner-party fight of the 1930s, Senior became an active supporter of the so-called "Militant" faction. After resigning his post late in 1936, Senior returned to graduate school, becoming a widely published academic specializing on the affairs of Puerto Rico and other nations of the Caribbean.