John Comyn III of Badenoch, nicknamed the Red (c. 1274 – 10 February 1306), was a leading Scottish baron and magnate who played an important role in the First War of Scottish Independence. He served as Guardian of Scotland after the forced abdication of his uncle, King John Balliol (r. 1292–1296), in 1296, and for a time commanded the defence of Scotland against English attacks. Comyn was stabbed to death by Robert the Bruce before the altar at the church of the Greyfriars at Dumfries.

His father, John Comyn II, known as the Black Comyn, had been one of the competitors for the Crown of Scotland, claiming his descent from King Donald III. His mother was Eleanor Balliol, sister of King John Balliol. He had, moreover, links with the royal house of England: in the early 1290s, he married Joan de Valence, cousin of King Edward I.

On the eve of the Wars of Independence, the Comyns were one of the dominant families of Scotland, with extensive landholdings in both the north and south of the country, and political influence and family connections with the crown. This Anglo-Norman family first made an appearance in Scotland during the reign of David I. In the thirteenth century they acquired the lordship of Badenoch, with extensive landholdings also in Lochaber, as well as the earldom of Buchan. On the death of Alexander III, John Comyn's father was appointed to the panel of Guardians to await the arrival of the infant Margaret, Maid of Norway, granddaughter of Alexander III. Her death in 1290 immersed the nation in crisis, finally solved in 1292 when John Balliol emerged as king, with the support of his Comyn kinsmen, a solution that was never accepted by the other main claimant, Robert Bruce of Annandale, grandfather of the future king. The Comyns were supporters of King John as was William Wallace.

With the outbreak of war between England and Scotland, Comyn, his father, and his cousin, John Comyn, Earl of Buchan, crossed the border and attacked Carlisle, defended for King Edward by Robert Bruce, Earl of Carrick, the father of the future king. The Wars of Scottish Independence thus began in a clash between the Bruces and Comyns. Having no siege equipment, the Comyns drew off and subsequently joined the main Scottish host at Haddington, which had been assembled to meet the advance of the English army along the east coast. On 27 April, the Scots were overwhelmed at the Battle of Dunbar, with John being among the many prisoners taken. While his father and cousin retreated north in the company of King John, he was sent south, to be imprisoned in the Tower of London.

John remained in prison for some months; but with the war in Scotland seemingly over he was finally released on condition that he take up service with Edward in Flanders, the main theatre of operations in his war against the French. While there he learned of the rising of William Wallace and Andrew Moray and their victory over the English at the Battle of Stirling Bridge. In March 1298, John was among Scots who deserted the English, finally ending up in Paris, where they appealed for aid to Philip IV of France. The only help they managed to get was a ship back to Scotland, arriving before the summer.

Earlier that year William Wallace emerged as Guardian after Moray died at Stirling or shortly after. The main task facing the Guardian was to gather a national army to meet an invasion by Edward, anxious to reverse the victory of Stirling Bridge. For cavalry, by far the weakest element of the Scottish host, Wallace depended on the Comyns and the other noble families. On 22 July Wallace's army was destroyed at the Battle of Falkirk, the light horse being driven off at an early stage by the heavy English cavalry.

It is possible that John Comyn was present at the battle, though the evidence is far from conclusive. The main Scottish sources, the chronicles of John Fordun and John Barbour, were composed decades after the event, long after the Comyns had been expelled from Scotland, and had a specific agenda, namely to magnify the later King, Robert the Bruce, and diminish John Comyn. According to Fordun, John and his kin hated Wallace and appeared on the battlefield only with premeditated treachery in mind – "For, on account of the ill-will, begotten of the sprig of envy, which the Comyns had conceived towards the aforesaid William, they, with their accomplices, forsook the field, and escaped unhurt." This is set alongside a commendation of Robert the Bruce, who, in Fordun's account, fought on the side of the English and "was the means of bringing about the victory." This is contested as no Bruce appears on the Falkirk roll of nobles present in the English army, and ignoring Blind Harry's 15th claim that Wallace burned Ayr Castle in 1297, two 19th-century antiquarians, Alexander Murison and George Chalmers, stated that Bruce did not participate in the battle and the following month decided to burn Ayr Castle to prevent it being garrisoned by the English. The contemporary English record of the Lanercost Chronicle simply blames the inadequacy of the Scottish cavalry in general. Soon after the defeat, John Comyn and Robert the Bruce were named as joint Guardians of the Realm in place of Wallace.

With no independent power base, Wallace, whose prestige had always been based on the success of his army, resigned or was removed as Guardian after Falkirk. In his place an unusual and difficult balancing act: John Comyn and Robert the Bruce, who had now joined the patriot party. The Scots were still fighting on behalf of the absent King John, so Bruce must have paid lip service to the cause, though his royal ambitions were openly known. The records give little or nothing in the way of insight into the feelings and motives of these men. At a meeting of a council of the magnates at Peebles in August 1299, an argument broke out relative to the property of Wallace, who was then in France. Comyn is said to have seized Bruce by the throat.