Nikhil Ranjan Sen (23 May 1894 – 13 January 1963) was an Indian-Bengali scientist who was a pioneer in the field of general relativity and called the father of applied mathematics in India. He received his PhD from Humboldt University of Berlin under the supervision of Max von Laue, thus becoming the first Indian to get a doctorate in relativity. Sen also worked on cosmogony, fluid dynamics, potential theory and probability, and founded the first fluid dynamics laboratory in India. He pioneered the study of ballistics and missiles in India and was also an advocate for science education in the Bengali language. His Bengali-language book titled “Soura Jagat” (The Solar System) was published by Visva-Bharati in 1949.