Sir Francis Harry Hinsley, OBE, FBA (26 November 1918 – 16 February 1998) was an English intelligence officer and historian. He worked at Bletchley Park during the Second World War and wrote widely on the history of international relations and British Intelligence during the Second World War. He was known as Harry Hinsley.

Hinsley's father worked in the coal department of the Walsall Co-Op. His mother Emma Hinsley (née Adey) was a school caretaker and they lived in Birchills, in the parish of St Andrew's, Walsall. Harry was educated at Queen Mary's Grammar School, Walsall and, in 1937, won a scholarship to read history at St. John's College, Cambridge. He went on to be awarded a first in part one of the Historical Tripos.

In August 1939, Hinsley visited his girlfriend in the German city of Koblenz. Police required him to report to the police station daily. However, this requirement was waived following the signing of the German-Soviet Pact. A week later, Hinsley was advised by police via his girlfriend's parents to get out of Germany by "tomorrow at the latest". This enabled him to cross the Franco-German border before it was closed. He made the crossing at the bridge between Kehl and Strasbourg. Stripped of his Reichsmarks by German border guards without francs or sterling in exchange, Hinsley was left penniless. This led to his sleeping on a park bench in France. Hinsley hitch-hiked to Switzerland from where he returned to the United Kingdom. He made his return just before Britain declared war on Germany. In October 1939, while still at St. John's, he was summoned to an interview with Alastair Denniston, head of the Government Code and Cypher School (GC&CS), and was thereby recruited to Bletchley Park's naval section in Hut 4. He abandoned his degree course and thereafter never completed it.

At Bletchley Park, Hinsley studied the external characteristics of intercepted German messages, a process sometimes termed "traffic analysis": from call signs, frequencies, times of interception and so forth, he was able to deduce a great deal of information about the structure of Nazi Germany's Kriegsmarine's communication networks and even about the structure of the German navy itself.

Hinsley helped initiate a programme of seizing Enigma machines and keys from German weather ships, such as the Lauenburg, thereby facilitating Bletchley Park's resumption of interrupted breaking of German Naval Enigma. He realised that, as the ships were on station for long periods, they would have to carry the code books (which changed every month) for subsequent months; these would likely be in a locked safe and might be overlooked when the crew threw Enigma materials (including the code book currently in use) overboard if the ship was boarded, an assumption which proved correct.

In late 1943, Hinsley was sent to liaise with the US Navy in Washington, with the result that an agreement was reached in January 1944 to co-operate in exchanging results on Japanese Naval signals.

Towards the end of the war, Hinsley, by then a key aide to Bletchley Park chief Edward Travis, was part of a committee which argued for a post-war intelligence agency that would combine both signals intelligence and human intelligence in a single organisation. In the event, the opposite occurred, with GC&CS becoming GCHQ.

On 6 April 1946, Hinsley married Hilary Brett-Smith, a graduate from Somerville College, Oxford, who had also worked at Bletchley Park, in Hut 8. They moved to Cambridge after the war where Hinsley had been elected a Fellow at St. John's College.