Gunnerius Ingvald Isachsen (3 October 1868 – 19 December 1939), was a Norwegian military officer and polar scientist.

His father was the shipmaster Nils Høgh Isachsen (1838–1913), and his mother was Marie Cecilie Sivertsen (1839–1909).

From 1898 to 1902, Isachsen was topographer on Otto Sverdrup's Fram expedition to the Arctic archipelago.

[2] During this voyage, he was promoted to Rittmester in 1899, and mapped large areas of hitherto unknown islands in Northern Canada, mainly by long sledge journeys.

In 1923, he became the Director of the Norwegian Maritime Museum in Oslo,[1][5] and in 1923 and 1924, he took part in expeditions to East Greenland .

He was the government's whaling inspector in the Southern Ocean in 1929–30, and the leader of the fourth Norvegia expedition circumnavigating the South Pole in 1930–31.