Marcus Calpurnius Bibulus was a politician of the late Roman Republic.

Bibulus was the son in law of Cato the younger. In 59 BC he was elected consul by the optimates, the conservative Republican enemies of Julius Caesar in the Senate. Caesar wanted his own ally Lucius Lucceius elected co-consul, but instead formed the First Triumvirate with Pompey and Crassus in order to circumvent the authority of Bibulus and the optimates.

Bibulus' only major act as consul was to veto the bill proposed by Caesar that would give land in Campania to Pompey's soldiers, and to then declare that the rest of the days on which the Centuriate Assembly could meet would be religious holidays. Caesar presented his bill at the Assembly anyway, and when Bibulus tried to intervene, the crowd broke his fasces and dumped feces on him. After this humiliation he refused to carry out the rest of his term, leaving Caesar with complete control over the consulship, although he occasionally issued complaints against Caesar, which led to attacks on his house from Caesar's supporters, the populares. For the rest of the year, the populares joked that the two consuls were really "Julius and Caesar."

As a senator in 52 BC, he supported Pompey, who was by now a political enemy of Caesar. Bibulus and his father-in-law, Cato the younger, allowed Pompey to serve as sole consul in 52 after the murder of Publius Clodius Pulcher. In 51 BC he became governor of Syria, but offended the army there by claiming a victory which had been won before he arrived.

In 48 BC he allied with Pompey against Caesar, commanding Pompey's navy in the Adriatic Sea. He captured Caesar's fleet, leaving Caesar stranded in Epirus, although this was a small feat as Caesar went on to defeat Pompey at the Battle of Pharsalus. Bibulus died later in 48.

Bibulus was married twice. From the first marriage he had three sons, including the later statesman Lucius Calpurnius Bibulus . His second wife was Cato's daughter Porcia Catones.