Aizawa Seishisai (会沢 正志斎, July 5, 1782 – August 27, 1863), born Aizawa Yasushi (会沢 安), was a Japanese nationalist thinker of the Mito school during the late shogunate period. In 1799 he became involved in the compilation of the Dai Nihon-shi (Great History of Japan) being undertaken by the Mito school. In 1840 Aizawa became the first head of professors of the Mito school's Kōdōkan but was forced to resign in 1844 when Tokugawa Nariaki resigned as domain leader. He later returned to the Kōdōkan.