Nicolae Constantin Paulescu (Romanian pronunciation: [nikoˈla.e pa.uˈlesku]; 30 October 1869 (O.S.) – 17 July 1931) was a Romanian physiologist, professor of medicine, and politician, most famous for his work on diabetes, including patenting pancreine (a pancreatic extract containing insulin). The "pancreine" was an extract of bovine pancreas in salted water, after which some impurites were removed with hydrochloric acid and sodium hydroxide. Paulescu was also, with A. C. Cuza, co-founder of the National Christian Union and later, of the National-Christian Defense League, an early ultranationalist and anti-Semitic Romanian party. He was also a leading member of the militant religious fascist Iron Guard.

Born in Bucharest, he was the first of four children of Costache and Maria Paulescu. He displayed remarkable abilities as early as his first school years. He learned French, Latin and Ancient Greek at an early age, so that a few years later he became fluent in all these languages and was able to read classical works of Latin and Greek literature in the original. He also had a particular gift for drawing and music and special inclinations towards natural sciences, such as physics and chemistry. He graduated from the Mihai Viteazul High School in Bucharest in 1888.

In the autumn of 1888, Paulescu left for Paris, where he enrolled in medical school, where he studied with, among others, Étienne Lancereaux. In 1897 he graduated with a Doctor of Medicine degree, with a research thesis on the structure of the spleen. At the same time he studied chemistry and physiology at the Faculty of Science of the University of Paris, and also obtained a doctorate in science.

Upon receiving his M.D., Paulescu was immediately appointed as assistant surgeon at the Notre-Dame du Perpétuel-Secours Hospital in Paris. In 1900, Paulescu returned to Romania, where he remained until his death (1931) as Head of the Physiology Department of the University of Bucharest Medical School, as well as a Professor of Clinical Medicine at the St. Vincent de Paul Hospital in Bucharest.

In 1916, he succeeded in developing an aqueous pancreatic extract which, when injected into a diabetic dog, proved to have a normalizing effect on blood sugar levels. Shortly after completing the experiments, he was called to service in the Romanian army. After World War I, he resumed his research.

From 24 April to 23 June 1921, Paulescu published four papers at the Romanian Section of the Society of Biology in Paris:

An extensive paper on this subject – "Research on the Role of the Pancreas in Food Assimilation" – was submitted by Paulescu on 22 June to the Archives Internationales de Physiologie in Liège, Belgium, and was published in the August 1921 issue of this journal.

The method used by Paulescu to prepare his pancreatic extract was similar to a procedure described by the American researcher Israel Kleiner in an article published two years earlier, in 1919, in the Journal of Biological Chemistry. Using his procedure, Kleiner had been able to demonstrate significant reductions in the concentration of blood and urinary glucose after intravenous injections of his extract.