Marcus Fusus (426-?) was a Roman veteran and agent during the 4th century BC. In 399 BC, he was a part of a conspiracy by Marcus Furius Camillus to assassinate the Veiian king Arnthi Spedo in Cisra, but Arnthi left the port before Fusus could do so. He was instead utilized as a scout, being sent far ahead of the Roman legions to spy on other nations.