A "Koryu", literally "old school", is an older martial tradition in Japan that was created before the Meiji Restoration in 1868. These older Japanese arts function very differently than the modern martial art (sometimes known as "Budo") that we are most familiar with. They are typically passed down within families to a "Soke" who function as the executive of the tradition and is tasked with ensuring its preservation. They feature extremely selective membership criteria and often require "keppan" (a "blood oath"), for the prospective student to prove their sincerity. The schools are often based on a founding principle, "Gokui", with a secretive or obscured source, that is only revealed on deeper initiation into the art, which is often passed down via "Mokoroku" scrolls. Instead of the familiar ranking and belts system of modern arts, they instead give licenses to selectively students to teach, known as "Menkyo", and further along, "Menkyo Kaiden". The oldest known "Koryu" practiced today is Tenshin Shoden Katori Shinto-ryu, which was founded in 1447.

Several "Koryu" including Kito-ryu and Tenjin Shinyo-ryu were incorporated into the modern martial art Judo by Jigoro Kano, which was intended to encourage interest in "Koryu" arts by Jigoro Kano. Judo students, in the formative stages of Judo, famously feuded with students of "Koryu" by which Judo solidified its popularity over these older forms of "jujutsu".