Christian Matthias Theodor Mommsen (/ˈmɒmzən/; German: [ˈteːodoːɐ̯ ˈmɔmzn̩] ^ⓘ; 30 November 1817 – 1 November 1903) was a German classical scholar, historian, jurist, journalist, politician and archaeologist. He is widely regarded as one of the greatest classicists of the 19th century. He received the 1902 Nobel Prize in Literature for his historical writings, including The History of Rome, after having been nominated by 18 members of the Prussian Academy of Sciences. He was also a prominent German politician, as a member of the Prussian and German parliaments. His works on Roman law and on the law of obligations had a significant impact on the German civil code.

Mommsen was born to German parents in Garding in the Duchy of Schleswig in 1817, then ruled by the king of Denmark, and grew up in Bad Oldesloe in Holstein, where his father was a Lutheran minister. He studied mostly at home, though he attended the Gymnasium Christianeum in Altona for four years. He studied Greek and Latin and received his diploma in 1837. As he could not afford to study at Göttingen, he enrolled at the University of Kiel.

Mommsen studied jurisprudence at Kiel from 1838 to 1843, finishing his studies with the degree of Doctor of Roman Law. During this time he was the roommate of Theodor Storm, who was later to become a renowned poet. Together with Mommsen's brother Tycho, the three friends even published a collection of poems (Liederbuch dreier Freunde). Based on his research into Roman law, Mommsen then devoted himself to Ancient History, which was just being established as a separate subject area at the time; thanks to a royal Danish grant, he was able to visit France and Italy to study preserved classical Roman inscriptions. During the revolution of 1848 he worked as a war correspondent in then-Danish Rendsburg, supporting the German annexation of Schleswig-Holstein and a constitutional reform. Having been forced to leave the country by the Danes, he became a professor of law in the same year at the University of Leipzig. When Mommsen protested against the new constitution of Saxony in 1851, he had to resign. However, the next year he obtained a professorship in Roman law at the University of Zurich and then spent a couple of years in exile. In 1854 he became a professor of law at the University of Breslau where he met Jakob Bernays. Mommsen became a research professor at the Berlin Academy of Sciences in 1857. He later helped to create and manage the German Archaeological Institute in Rome.

In 1858 Mommsen was appointed a member of the Academy of Sciences in Berlin, and he also became professor of Roman History at the University of Berlin in 1861, where he held lectures up to 1887. Mommsen received high recognition for his academic achievements: foreign membership of the Royal Netherlands Academy of Arts and Sciences in 1859, the Prussian medal Pour le Mérite in 1868, honorary citizenship of Rome, elected a member of the American Antiquarian Society in 1870, and the Nobel Prize in Literature in 1902 for his main work Römische Geschichte (Roman History). (He is one of the very few non-fiction writers to receive the Nobel Prize in Literature.)

In 1873, he was elected as a member of the American Philosophical Society.

At 2 a.m. on 7 July 1880 a fire occurred in the upper floor workroom-library of Mommsen's house at Marchstraße 6 in Berlin. After being burned while attempting to remove valuable papers, he was restrained from returning to the blazing house. Several old manuscripts were burnt to ashes, including Manuscript 0.4.36, which was on loan from the library of Trinity College, Cambridge. There is information that the important Manuscript of Jordanes from Heidelberg University library was burnt. Two other important manuscripts, from Brussels and Halle, were also destroyed.

Mommsen had sixteen children with his wife Marie (daughter of the publisher and editor Karl Reimer of Leipzig). Their oldest daughter Maria married Ulrich von Wilamowitz-Moellendorff, the great Classics scholar. Their grandson Theodor Ernst Mommsen (1905–1958) became a professor of medieval history in the United States. Two of the great-grandsons, Hans Mommsen and Wolfgang Mommsen, were German historians.

While he was secretary of the Historical-Philological Class at the Berlin Academy (1874–1895), Mommsen organised countless scientific projects, mostly editions of original sources.