Marcus Fabius N.f. Ambustus (fl. 360–351 BC) was a statesman and general of the Roman Republic. He was the son of Numerius Fabius Ambustus.
He served as consul three times: in 360, 356, and 354 BC. His consulships occurred during a time in which Rome was reasserting itself following its defeat at the hands of the Gauls in the Battle of the Allia of 387 BC. He defeated the Hernici in 356, and Tibur in 354, earning a triumph for the latter victory. He further succeeded against the Falisci, but was defeated by Tarquinia.
As he was absent from Rome when the time came for holding the comitia, the senate, which did not like to entrust them to his colleague, who had appointed a plebeian dictator, and still less to the dictator himself, nominated interreges for the purpose. The object of the patricians was to secure both places in the consulship for their own order again, which was effected by Ambustus, who seems to have returned to Rome in the meantime. He was appointed the eleventh interrex in 355, and declared two patricians consuls in violation of the Licinian law (the plebs had been made eligible for the consulship again, over his objections), although he was not successful in his object. He served as interrex again in 351, and as dictator in 351. There is some disagreement between scholars over whether he ever served as censor, possibly in 358, and as princeps senatus later in life (the two questions are connected, as all principes prior to 209 BC were censorii).
He was alive in 325, when his son, Quintus Fabius Maximus Rullianus, was Master of the Horse to Lucius Papirius Cursor, and fled to Rome to implore protection from the vengeance of the dictator. He interceded on his son's behalf both with the senate and the people. He was also father of Marcus Fabius Ambustus, who also served as Master of the Horse.