Cort Sivertsen Adeler (16 December 1622 – 5 November 1675), known in Denmark as Coort Sifvertsen Adelaer, in the Netherlands as Koert Sievertsen Adelaer and in Italy as Curzio Suffrido Adelborst, was the name of honour given to Kurt Sivertsen, a Norwegian seaman, who rendered distinguished service to the Danish and Dutch navies, and also to the Republic of Venice against the Turks.

At the age of fifteen he took service with the Dutch navy; in 1639 he fought under Lieutenant-Admiral Maarten Tromp at the Battle of the Downs.

In 1642 he was first mate on the Grote St. Joris, a Dutch ship hired by the fleet of Venice as the San Giorgio Grande.

His son Sivert Adelaer served as a cadet on the ship of the famous Dutch Vice-Admiral Michiel de Ruyter.

After the death of the Dutch supreme commander Lieutenant-Admiral Jacob van Wassenaer Obdam in the Battle of Lowestoft Adelaer was considered for this function but again indicated that he had no interest in it.

Subsequently, Adelaer was asked to join the Danish-Norwegian navy as operational supreme commander, to supervise the modernisation of their fleet.

Adeler acquired a substantial private capital and owned among others estates Dragsholm in Denmark and Gjemsø Kloster and Bratsberg near the city of Skien in Norway.

[3] Adelaer was a personal friend of the new Dutch supreme commander Lieutenant-Admiral Michiel de Ruyter who also had been knighted in Denmark, for his victory over Sweden in 1659.