Ryutaro Nagai (?? ???, Nagai Ryutaro, April 16, 1881 – December 4, 1944), was a politician and cabinet minister in the Empire of Japan, serving a member of the Lower House of the Diet of Japan eight times, and four as a cabinet minister.
He was noted in his early political career as a champion of universal suffrage, social welfare, labor unions, women's rights and Pan-Asianism.