Knagenhjelm, also spelled Knagenhielm and in the United States Knagenhelm, is a Danish and Norwegian noble family living in Norway. It descends from Niels Tygesen Knag (1661–1737), who owned Kaupanger Head Farm. The name Knagenhielm was granted when he on 21 December 1721 was lifted up in the Dano-Norwegian nobility in recognition of his efforts to stave off Russian encroachment on Norwegian territory. Descendants still own Kaupanger, and are included in the Yearbook of the Danish Nobility.
Niels Tygesen Knag was the son of district magistrate Tyge Nilsen Castberg, allegedly from Jutland, and Maren Nielsdatter Knag, the daughter of Niels Knag, district magistrate on Sunnmøre. Even the Norwegian family Castberg descends from this couple.
The coat of arms was received upon the ennoblement in 1721.
Description without the heraldic terminology: On a blue shield a tilted, chopped and knotty silver tree trunk beside or above which there are three golden stars in each of two bends. On the helmet is a six-pointed star between two buffalo horns divided in gold and blue, vice versa, respectively.
Some family members use also an oak leaf on the tree trunk.
Before he became noble, Niels Tygesen Knag had only a tree trunk as his arms.