American Communist Party leader and labour organizer. He was general secretary of the Communist Party of the United States of America (CPUSA) from 1945 to 1957 and national chairman during 1959–61.

Having worked at various trades in Seattle, Dennis joined the Industrial Workers of the World and in 1926 the Communist Party. Later he went to California to organize workers, for which he was arrested numerous times. Jumping bail after a conviction in 1930, he spent several years in the Soviet Union and other countries. Returning to the United States in 1935, he began to rise in the hierarchy of the Communist Party. In 1949 he and 10 other Communist leaders were convicted of conspiring to advocate the violent overthrow of the government, for which Dennis served three years and nine months in prison. During his leadership of the Communist Party, Dennis supported the party's continued alignment with the Soviet Union.