The National Diet Building is the building where both houses of the National Diet of Japan meet. It is located at Nagatachō 1-chome 7–1, Chiyoda, Tokyo.

The National Diet is the national legislature of Japan. It is composed of a lower house, called the House of Representatives, and an upper house, the House of Councillors. Both houses are directly elected under a parallel voting system. In addition to passing laws, the Diet is formally responsible for nominating the Prime Minister. The Diet was first established as the Imperial Diet in 1890 under the Meiji Constitution, and took its current form in 1947 upon the adoption of the post-war constitution. Both houses meet in the National Diet Building in Nagatachō, Chiyoda, Tokyo.