Nikolaj Eeg Kruse Hartz (August 23, 1867 in Randers – May 7, 1937) was a Danish geologist and botanist. He investigated plant and animal remains in deposits from before, during and after the last glaciation. Together with his colleague Vilhelm Milthers, he investigated a clay pit near Allerød in Denmark and found that a period with milder climate and birch forest – the Allerød oscillation - had interrupted the cold and dry Dryas stadial (Hartz & Milthers 1901). He also was an important explorer of the flora of Greenland during several expeditions.
He studied botany at the University of Copenhagen. He graduated in 1895 and obtained a Ph.D. in 1909 with a dissertation about the late-glacial flora and fauna of Denmark. From 1896, Hartz was a researcher at the Geological Survey of Denmark. In 1913, he abandoned his scientific career and entered his brothers private company.
The grass species Poa hartzii was described by the French botanist Michel Gandoger (1920), who named it after Hartz because it was based on plants collected by Hartz in "Kordlunguak" at the Sullorsuaq Strait, West Greenland.