The paper summarizes the life and work of the important Czech legal historian Jiří Kejř, focusing on the period of his twenty-year tenure at the Institute of State and Law of the Czechoslovak Academy of Sciences. His research interests focused on the Czech Middle Ages, specifically on the administrative and legal history of the Hussite period, especially the history of medieval cities, the University of Prague, the personality of Master Jan Hus and codicology. Moreover, Jiří Kejř authored a number of analytical studies and several synthesizing monographs. His works were based on a thorough study of manuscripts. His works from periods of oppression of freedom bear no trace of the ideological pressure prevailing at the time of their creation. They still have a high scientific value, so other researchers cannot neglect his work.