Karl Joachim Marquardt (19 April 1812 – 30 November 1882) was a German historian and writer on Roman antiquities.
Marquardt was born at Danzig (Gdańsk).
He studied at Berlin and Leipzig, held various educational appointments from 1833 onwards at Berlin, Danzig and Posen (Poznań), and became in 1859 head of the gymnasium in Gotha, where he died on in 1882. The dedication of his treatise Historiae equitum romanorum libri quatuor (1841) to Lachmann led to his being recommended to the publisher of Wilhelm Adolf Becker's Handbuch der römischen Alterthumer to continue the work on the death of the author in 1846.
It took twenty years to complete, and met with such success that a new edition was soon called for. Finding himself unequal to the task single-handed, Marquardt left the preparation of the first three volumes (Römisches Staatsrecht) to Theodor Mommsen, while he himself contributed vols. V-VI (Römische Staatsverwaltung, 1873–1878) and vol. VII (Das Privatleben der Römer, 1879–1882).
Its clearness of style, systematic arrangement and abundant references to authorities ancient and modern, will always render it valuable to the student.
Marquardt died in Gotha.