Marc-Guillaume Alexis Vadier (17 July 1736 – 14 December 1828) was a major French politician of the French Revolution. He is sometimes called the "Great Inquisitor", for his active participation in the Reign of Terror.

He is primarily known for having created the département of Ariège and mostly for having firmly led the Committee of General Security, thus being one of the key figures of the Reign of Terror and the "dean of its political police." Vadier also had an influential role on 9 Thermidor, during the fall of Robespierre, with whom he had a long-standing rivalry.

He managed to avoid reprisals for his role during the Terror, which targeted him following Robespierre's fall, and survived the Revolution, participating in relative anonymity during the First Empire period. He was later exiled under the law targeting regicides during Louis XVIII's reign.

Like Robespierre, Barère, or Billaud-Varenne, his exact role during the Terror is still debated by historians. However, it is acknowledged that a significant part of the repressive measures of the Terror and the executions can be attributed to the Committee of General Security, which he led, and that he participated in organizing the Great Terror by increasing the number of executions alongside Fouché, notably to accuse Robespierre.

Son of a wealthy family in Pamiers, in the County of Foix (now Ariège), he served in the army of King Louis XV, taking part in the Seven Years' War and the Battle of Rossbach on 5 November 1757. Upon his return to France in 1758, Vadier acquired large tracts of land in Pamiers and in 1770 purchased the office of conseiller (magistrate), which brought him into conflict with many of the local aristocracy and affluent bourgeoisie. Elected as deputy to the Third Estate in the Estates-General of France for the County of Foix (in 1789), Vadier took no prominent part in that assembly.

He left his office as representative in the National Constituent Assembly of the Constitutional Monarchy (September 1791), returning to Ariège département, becoming president of the district tribunal in Mirepoix, contributing to the creation of new revolutionary institutions throughout the region. With the overthrow of the monarchy one year later, (10 August 1792), he was elected to the National Convention (3 September), sat among the Montagnards, joined the Jacobin Club, and voted in favor of Louis XVI's execution (sans appel ni sursis – without appeal or delay) on 17 January 1793. An opponent of the Girondists, Vadier opposed the proscription of Jean-Paul Marat, and following the fall of the Girondists travelled to Ariège to actively oppose the Federalist Revolt there. Vadier was an unwavering and fanatical Jacobin, known for his particular violence, often traveling in Ariège with armed men by his side. He also stood out due to his Gascon accent.

From September 1793 onward, he became the president and dean of the Committee of General Security, the policing and repressive organ of the Terror, earning the title of the "dean of the political police" during the Terror, according to Olivier Blanc. Vadier quickly fell out with Robespierre, particularly due to his militant atheism and the creation of the General Police Bureau by the Committee of Public Safety, while he sought to control repression. Under his leadership, the Committee of General Security became a "ministry of Terror."

In Nivôse Year II (January 1794), along with Amar, he exposed the wrongdoings of Fabre d'Églantine, Chabot, and their associates, including Danton, and initiated the trial targeting the Dantonists. He referred to Danton as a "stuffed turbot" in the course of his attacks. It seems that he and Amar were the main instigators of the trial of the Dantonists. He directly approached Fouquier-Tinville, requesting him to have certain accused individuals guillotined.