Robert Jacobus Fruin (11 November 1823 in Rotterdam – 29 January 1899 in Leiden) was a Dutch historian, who as a follower of Leopold von Ranke introduced the scientific study of history in the Netherlands, when he was professor of Dutch national history (Dutch: Vaderlandsche Geschiedenis) at Leiden University.
Fruin, a lifelong bachelor, studied classical philology at Leiden University from 1842, and received his Ph.D. on 18 December 1847 with a dissertation on Manetho, entitled De Manethone Sebennytha. As he was a man of independent means, he spent the next two years in independent studies and political pursuits in Utrecht, before he accepted a position as praeceptor (teacher) in history at Leiden Gymnasium in 1850.
These were tumultuous times in Dutch constitutional history as the liberal 1848 Dutch constitutional reform by Johan Rudolf Thorbecke had just been completed, and was the subject of heated political debate. In this debate Fruin took the liberal side and he conducted a learned polemic with a fellow eminent Dutch historian, who was also a leader of the Conservative opposition to the new political ideas, Guillaume Groen van Prinsterer. Though the polemic was courteous, it would inaugurate a certain coolness in the professional and personal relationships between the two men, that would last during their careers.
They would cross swords in their professional fields also. Groen van Prinsterer had a style of historiography that was more poetic than Fruin thought appropriate. He shared that style with the American historian John Lothrop Motley who would use his archival work on William the Silent in his own work on the Dutch Republic: The Rise of the Dutch Republic, which caused a furore in the Netherlands. Though Fruin had much sympathy for Motley's work, he also was prompted by it to do his own original research on the events leading up to the Dutch Revolt, and on the crucial years of that Revolt, which led him to completely different conclusions. He published these in Het Voorspel van den Tachtigjarigen oorlog (Prologue to the Eighty Years' War) and Tien jaren uit den Tachtigjarigen oorlog (Ten years from the Eighty Years' War).