Gunnar Fridtjof Thurmann Sønsteby DSO (11 January 1918 – 10 May 2012) was a member of the Norwegian resistance movement during the German occupation of Norway in World War II.

Norway's regular armed forces surrendered on 10 June 1940, after two months of fighting, and the country was subsequently occupied by the Germans.

[6] In 1941 he was brought into the secret British military unit called Special Operations Executive (SOE) at their office in Stockholm.

While on assignment in Stockholm in 1942, he was interned and imprisoned for three months by Swedish police, but managed to convince them that he was not the same Gunnar Sønsteby for whom they were looking.

From there, he was sent to Britain, where in June of that year he enrolled in the Norwegian Independent Company 1, known in Norway as Kompani Linge or Linge Company, which was formed to participate in British-led operations in Norway, to organise, instruct, and lead the Norwegian Resistance Movement, to serve as a link between the home front and the outside world, and to perform intelligence work.

In that same month he also became head of the newly established Oslo Gang, a sabotage group, whose other members were Andreas Aubert, Viggo Axelsen, Gregers Gram, Henrik Hop, William Houlder, Max Manus, Martin Olsen, Arthur Pevik, Birger Rasmussen, Tor Stenersen, and Edvard Tallaksen.

After saboteur training in England in 1943, he became the contact for all SOE agents in eastern Norway and head of the Norwegian Independent Company 1 group in Oslo.

In addition, they killed several leading figures in the Occupation Forces, including the Nazi head of police in Norway, Karl Marthinsen.

[10] After D-Day, Sønsteby concentrated largely on bombing Norwegian railways, thereby keeping German reinforcements from being moved back to the front line.

His obituary in Aftenposten attributed his ability to elude capture to "resourcefulness, luck, intuition," and "such an entirely ordinary appearance that he was hardly noticed when he rode his bicycle through Oslo's streets.

"[13] The Nazis withdrew from Norway on 8 May, and on 13 May Sønsteby led the procession when Crown Prince Olav, the first member of the royal family to return from exile in London, arrived in Oslo.

Instead, in the autumn of 1945, Sønsteby moved to Boston, where he worked at a government purchasing center in New York and took part in an executive study program at Harvard Business School.

Harald Stanghelle wrote in 2018 that Sønsteby was for many years a "living war encyclopedia" who helped serve as "an effective political vaccine against all forms of fascism" and who, while "factually oriented and sober," could get angry in debates at persons who tried to equate democracies with autocracies and had little patience for "historyless historians and ignorant journalists."

At the same time, he minimised his own personal contributions to the war effort, saying that the merchant marines had played a more significant role in Norway's fight against the Nazi occupiers.

The pallbearers were six officers, a break from the norm of using soldiers drafted from the King's Guard, and as they carried the coffin from the church, four Air Force F-16s performed a missing man flypast.

The winners have included Per Edgar Kokkvold and Kristin Solberg (2015), Deeyah Khan and Trond Bakkevig (2016), and ten Norwegian veterans (2017).

Sønsteby's service during the Second World War is depicted in the film Number 24  [no] which premiered in Norway in October 2024 and began streaming on Netflix in January 2025.