Brigadier John Hessell Tiltman, CMG, CBE, MC (25 May 1894 – 10 August 1982) was a British Army officer who worked in intelligence, often at or with the Government Code and Cypher School (GC&CS) starting in the 1920s.

It was to exploit those methods, at extremely high speed with great reliability, that Colossus, the first digital programmable electronic computer, was designed and built.

After a decade as a War Office civilian at GC&CS, the interwar cryptographic organization, Tiltman was recalled to active service.

[8] He assisted in many areas of endeavour at GC&CS and was considered one of Bletchley Park's finest cryptanalysts on non-machine systems.

In 1951 Tiltman met William Friedman, one of the leading scholars involved in the attempt to decipher the mysterious Voynich manuscript.

Tiltman undertook an analysis of the ancient manuscript himself, and then in the 1970s he assigned an NSA cryptanalyst named Mary D'Imperio to take over the Voynich crypto-analysis, when Friedman's health began becoming a challenge.