The Tatenokai (楯の会, 楯の會) or Shield Society was a private militia in Japan dedicated to traditional Japanese values and veneration of the Emperor. It was founded and led by the author Yukio Mishima.
The Tatenokai was created on October 5, 1968, recruiting its membership primarily from the staff of Ronso Journal, an obscure right-wing college newspaper. In an unusual move, the Tatenokai was granted the right to train with the nation's armed forces, the Japan Self-Defense Forces. On November 25, 1970 Mishima and four Tatenokai members briefly seized control of the Self-Defense Force's headquarters and attempted to rally the soldiers to stage a coup d'état and restore imperial rule. When this failed, Mishima and Masakatsu Morita, the Tatenokai's main student leader, committed seppuku (ritual suicide). The rest of the members, around 90 people, were not informed about Mishima's plan at all.
On 3 March 1977, four Japanese nationalists took 12 hostages at the Keidanren Kaikan (headquarters and hall of Japan Federation of Economic Organizations), spreading leaflets at the scene that denounced big business. The hostages were released, unharmed, after an eleven-hour standoff during which the hostage-takers spoke for more than three hours to Mishima's widow, Yoko. Two of the hostage-takers – Yoshio Ito and Shunichi Nishio – were believed to have been former members of the Tatenokai.