Nils Claus Ihlen (24 July 1855 – 22 March 1925) was a Norwegian engineer and politician for the Liberal Party.

He was born in Skedsmo as the oldest son of Wincentz Thurmann Ihlen (1826–1892) and Birgitte Elisabeth Mørch (1830–1913).

[1] Ihlen took his education at the Eidgenössische Technische Hochschule of Zürich and returned to Norway in 1877 to work one year for the Norwegian State Railways.

In June 1920, Ihlen resigned along with the rest of the second cabinet Knudsen and then left national politics.

The question eventually went all the way to the Permanent Court of International Justice in the form of the Eastern Greenland Case in 1933.