Nikolaus Gjelsvik (11 April 1866 –14 November 1938) was a Norwegian jurist and law professor.
He was born at Vevring Municipality in Sunnfjord. He served as a professor at the University of Kristiania (now University of Oslo) from 1906.
Among his works are the books Innleiding i rettsstudiet from 1912, Lærebok i folkerett from 1915, and Lærebok i millomfolkeleg privatrett from 1918. He was a proponent for the Nynorsk language, took part in organisational work, and had leading positions in the publishing house Det Norske Samlaget and in the societies Noregs Ungdomslag and Noregs MÃÂ¥llag.
Marshals at his funeral were Nicolai Rygg and Ferdinand Schjelderup.