Defunct The Rikken Seiyūkai (立憲政友会, Association of Friends of Constitutional Government) was one of the main political parties in the pre-war Empire of Japan.

The Seiyūkai was the most powerful political party in the Lower House of the Diet of Japan from 1900 to 1921, and it promoted big government and large-scale public spending.

While in the opposition during the Minseitō-dominated cabinet of Prime Minister Hamaguchi Osachi, the Seiyūkai attacked the ratification of the London Naval Treaty of 1930 as against Article 11 of the Meiji Constitution, which stipulated the independence of the military from civilian control.

After winning the 1932 General Election under Inukai Tsuyoshi, Seiyūkai formed a cabinet, floated the yen and conducted policies to revive the economy.

In 1940, it voted to dissolve itself into the Imperial Rule Assistance Association as part of Fumimaro Konoe's efforts to create a one-party state, and thereafter ceased to exist.