Petrus Johannes Blok (Den Helder, 10 January 1855 – Leiden, 24 October 1929) was a Dutch historian.
Born in Den Helder, Blok studied at the Latin School of Alkmaar and read classics at Leiden University, receiving his doctorate for a study of Sextus Pompeius. After this, he got a position at the Leiden Latin School, and published two books on the city's Medieval and Burgundian history.
In 1884, he was made professor of Dutch History at the University of Groningen, where he supervised the publication of a series of historical documents from the provinces of Groningen and Drenthe. He was in the habit of traveling much to search for written documents, which brought him as far as Rome: a very long trip back then. Among his students was Pieter Jelles Troelstra.
In 1892, Blok was elected a member of the Royal Netherlands Academy of Arts and Sciences. In 1894, he succeeded Robert Fruin as professor of Dutch History in Leiden.
Blok believed that it was the task of a historian to stress a country's national past and contribute to a sense of unity. It does not come as a surprise that he was one of the editors of the Nieuw Nederlandsch Biografisch Woordenboek ("New Dutch Biographical Dictionary"). His books are biographical in nature and are about Dutch heroes: Oranje (1919–1920), Frederik Hendrik (1924), and De Ruyter (1928).