Marcus Fabius Ambustus was Magister Equitum of the Roman Republic in 322 BC. The identification of him as the son of the consul M. Fabius Ambustus depends on a reference in Livy to the active military service of a cavalry officer serving under the dictator Aulus Cornelius Cossus Arvina, but T.R.S. Broughton finds it more likely that the three-time consul was himself the Magister Equitum carrying out administrative duties.