Matthias Gelzer (19 December 1886, Liestal – 23 July 1974, Frankfurt am Main) was a Swiss-German classical historian, known for his studies of the Roman Republic in regard to its politics and society. He was the author of highly regarded biographies on Julius Caesar, Pompey and Cicero.
He studied history and classical philology at the universities of Basel and Leipzig, where in 1909 he received his doctorate as a student of Ulrich Wilcken. In 1912 he obtained his habilitation at the University of Freiburg with a thesis on the nobility of the Roman Republic. In 1915 he became a professor of ancient history at the University of Greifswald, and in 1918 relocated as a professor to the University of Strasbourg. From 1919 to 1955 he was a professor of ancient history at the University of Frankfurt am Main, where he served as its rector in 1924/25.
From 1925 to 1962 he was on the editorial board of the journal "Gnomon", for which he contributed numerous reviews. He was also the author of several entries in the Pauly-Wissowa "Realencyclopädie der classischen Altertumswissenschaft".