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

Fairbairn was a black belt of the Kodokan, 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. Fairbairn published a book, "Defendu", in 1926 (re-printed as "Scientific Self Defence" in 1931), illustrating this method - and it is here that the term "Defendu" first appeared.

This confused early readers of the book, who assumed that the techniques within had some direct basis in the Eastern martial arts that Fairbairn had learned. Thus, in an attempt to highlight the originality of Fairbairn's material, the term did not appear in the 1931 edition of the book.

Fairbairn was called upon by the British to help train Allied troops in WWII, by which time 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 original Defendu was oriented towards self defense and restraint, while the Close Quarters Combat system concentrated on rapid disabling of an opponent, with potentially lethal force. The militarized version of Defendu is described in Rex Applegate's book "Kill or Get Killed".

Fairbairn published several more books on the subject of self defense, all of which refer to Defendu only in relation to the earlier book. "Gutter fighting" or "the Fairbairn system" appeared to be his preferred terms for the style.