Defendu is a modern martial art developed by William E. Fairbairn. Defendu is a hand-to-hand combat system based on judo that was developed to train the Shanghai Municipal Police, and was later taught in expanded for to OSS and SOE members during WWII.

Fairbairn was a black belt of the Kodokan Institute, where he learned judo while working as an instructor and head of the Shanghai Municipal Police police riot squad between the years 1907 and 1940. Based on this training, and actual fights he was involved in during his police work, Fairbairn began to develop his own system of hand-to-hand combat, Defendu. It was designed to be simple to learn and brutally effective. By the time Fairbairn was called back to help train Allied troops in WWII, Defendu had a well established record. Fairbairn and others expanded on this system to create the Close Quarters Combat system that was then taught to the troops. This system was built on Defendu, but modified for military applications, rather than police and riot control. The orignal Defendu was oriented towards self defense and restraint, while the Close Quarters Combat system concentrated on rapid disablinng of an opponent, with potentially lethal force. The militarized version of Defendu is described in Rex Applegate's book "Kill or Get Killed".