Malcolm III [Mael Coluim Ceann Mór, Malcolm Canmore] (d. 1093), king of Scots, was the eldest son of Duncan I, king of Scots from 1034 to 1040, and his wife, who is said to have been a cousin of Siward, earl of Northumbria (d. 1055). During the reign of Macbeth (r. 1040–57), who had killed Duncan I in order to take the throne, Malcolm was an exile at the court of Edward the Confessor, king of England, and was given a small estate in Northamptonshire. In 1054 Malcolm was present with an army led into Scotland by Siward, probably by command of King Edward, who supplied some of Siward's forces. On 27 July the earl's army defeated Macbeth, traditionally on Dunsinnan Hill in the Sidlaws, with heavy losses on both sides. The victory evidently put Malcolm in possession of Scotland south of the Tay. Three years later (on 15 August 1057) he slew Macbeth at Lumphanan, in what is now Aberdeenshire. Although Macbeth's stepson, Lulach [see under Macbeth], was accepted as king by some of the Scots (he can hardly have been made king at Scone, the traditional place for royal inaugurations), Malcolm ambushed and killed him near Rhynie in Strathbogie, in March 1058, and (on 25 April, according to the fourteenth-century historian John Fordoun) succeeded to the throne.

In spite of a peaceful visit to Edward the Confessor in 1059, the first of Malcolm's five raids into English Northumbria took place in 1061, when Holy Island was plundered. Since 1018 at the latest, Scottish Northumbria (that is, Lothian and the merse of Berwickshire) had been part of the northern kingdom. Malcolm seems to have been determined to annex Northumbria between Tweed and Tees, his ambition forming the mirror image of that of the West Saxon kings of England, who regarded the territory from Tees to Forth as part of their lordship. In contrast to the experience of his predecessor Macbeth, Malcolm's relations with the Scandinavian rulers of the northern isles were friendly. About 1060 he married, as his first wife, Ingibjorg (d. c.1067), who was either the widow or the daughter of the earl of Orkney, Thorfinn, son of Sigurd. She is more likely to have been the earl's daughter: Thorfinn's death is usually put at 1064 or 1065, and with his first wife Malcolm had three sons, Duncan II, Donald, and possibly Malcolm, for whose births there would scarcely have been time between the mid-1060s and the king's second marriage, in either 1069 or 1070.

This second, and famous, marriage, to Margaret (d. 1093), daughter of Edward Ætheling (d. 1057) and great-niece of Edward the Confessor, was a consequence, however remote, of the Norman conquest of England. In 1066, following the Confessor's death and Harold Godwineson's assumption of the English throne, Tostig, Harold's brother and until 1065 earl of Northumbria in succession to Siward, plotted with the count of Flanders, Harald Hardrada, king of Norway, and Malcolm, king of Scots, to overthrow Harold Godwineson and, probably, to partition England between them. Although during the summer of 1066 Tostig had been sheltered by King Malcolm, and was said to have been his 'sworn brother', the Scots played no part in the sea-borne invasion of northern England carried out by Tostig and the king of Norway, which ended disastrously for both leaders on 25 September at the battle of Stamford Bridge. Malcolm seems to have viewed the confusion which followed William of Normandy's victory at Hastings as an opportunity to fulfil his ambitions in Northumbria south of Tweed. Although the chronology of events from 1068 to 1071 is hard to establish with precision, it is clear that English resistance to the Normans looked to Scotland for refuge and military support. In the summer of 1068 Edgar Ætheling, upon whom the aspirations of English nationalists were focused, fled to the Scottish court with his mother and his two sisters, Margaret and Christina. A northern English rising was thwarted by King William's occupation of York and an agreement made with Malcolm which staved off a Scottish invasion. By 1070, however, Malcolm was raiding south of Tweed with a large army. It is not likely that this incursion was intended to support the second Northumbrian rising, which had begun with the killing of Robert Comin (whom William I had appointed earl of Northumbria) in January 1069, for that rising had been effectively suppressed by the Conqueror by Easter 1070. Nevertheless, it was in the period 1069–70 that Malcolm married Edgar Ætheling's sister Margaret. The marriage cannot have been viewed by William I as a friendly act, and must have been meant to give any heirs of the Scottish king's second marriage a claim to English kingship.

Malcolm's aggressive policy led to the Conqueror's only expedition to Scotland. In 1072 William led an army and fleet north as far as the Firth of Tay, and received the homage of the king of Scots at Abernethy. Probably as a result of this submission, Edgar Ætheling made his peace with William, who treated him honourably. The dynasty ruling in Moray still had leaders who could challenge Malcolm's kingship, for in 1078 Lulach's son Mael Snechta was in rebellion, and Malcolm captured his mother and chief adherents. A reconciliation may have followed, as Mael Snechta died peacefully in 1085. In the meantime, the king of Scots carried out his third major raid into Northumberland (1079), sparing the lands and church of Hexham out of reverence for the local saints. The Conqueror's response to this was to send north his eldest son, Robert Curthose, with an army (1080), but he got no further than Falkirk and achieved little beyond building the ‘New Castle’ on the north bank of the Tyne, near the site of the Roman bridge (pons Aelius). That Malcolm was sensitive in the matter of his authority over Scottish Northumbria is shown by a story told in Symeon of Durham's Historia ecclesiae Dunelmensis. According to this account, Turgot (subsequently Queen Margaret's biographer and bishop of St Andrews), in association with Aldwin, formerly a monk of Winchcombe in Gloucestershire, attempted to revive monastic life at Old Melrose, perhaps in the late 1070s. The Scottish king demanded that the two English monks take an oath of fealty to him, and when this was refused persecuted them in various ways, until at last they were recalled to England by Bishop Walcher of Durham (d. 1080).

For the rest of William I's reign Malcolm's relations with England seem to have been peaceful, and at the accession of William Rufus, Malcolm's eldest son, Duncan, who had been for years a hostage in England, was knighted by the new king and allowed to go to Scotland in freedom. But by 1091 Malcolm was once again raiding Northumbria as far south as Durham. A sea and land expedition mounted by Rufus was severely hampered by bad weather which sank his fleet and played havoc with his cavalry force. Robert Curthose and Edgar Ætheling negotiated a peace which restored English estates to Malcolm which had been given by the Conqueror and, according to some sources, involved Malcolm's homage to Rufus. The latter seems to have been impressed by the fragility of the Scottish border, for in 1092 he went north again and forcibly annexed Carlisle and its district, building a strong castle and settling the area with English peasants. It has been suggested that there had been recent precedent for an English take-over of at least part of Cumbria, inasmuch as Earl Siward is shown exercising authority over a free tenant west of Carlisle, in a writ issued c.1050 by an unidentified lord named Cospatric. The authenticity of this writ is uncertain, but even if it is genuine it deals only with a restricted area between Carlisle and Wigton, and can scarcely be regarded as evidence that Siward held territorial lordship even over Cumbria south of the Solway. A statement in the Historia regum of Symeon of Durham, that in 1070 Cumberland was under Malcolm's control, 'not possessed by right but subjugated by force' (Symeon of Durham, Opera, 2.191), shows both that the region was then being ruled by the Scottish king, and that there were Englishmen who resented this. The status of Cumberland had in fact long been disputed, so that Rufus's seizure of Carlisle and its district hardly needed any precedent, but such an act of aggression nevertheless gave Malcolm a clear grievance.

It was agreed that Malcolm would meet the English king at Gloucester at the end of August 1093. On his journey south he took part in the ceremony of laying the foundation stones for the new (that is, the present) cathedral at Durham. But when the Scottish king reached the English court, William disdainfully refused to see him. An enraged Malcolm (having visited his elder daughter Edith at Wilton) returned to his own land and at once mustered a force with which to inflict maximum damage upon English territory. North of Alnwick he and Edward, the eldest son of his second marriage, were ambushed by the earl of Northumbria, Robert de Mowbray, and were both killed, the king falling victim to Archil Morel of Bamburgh, described as his 'comrade', on 13 November 1093. Malcolm was buried later the same month at Tynemouth Priory and a younger son, Edgar, carried the news of the death of his father and brother to Edinburgh where his mother, Queen Margaret, lay ill, exhausted by her too severe regime of self-denial. She died of grief on 16 November and was buried in the monastic church she had founded at Dunfermline, where in the reign of her son Alexander I (r. 1107–24) the body of her husband was brought for burial beside her. At the Reformation the remains of both Margaret and Malcolm were transferred by Philip II of Spain to the Escorial at Madrid.

In spite of the brutality involved in his raids into English Northumbria, Malcolm III left a reputation for sagacity, and a story preserved by Ailred of Rievaulx, in which Malcolm placed himself in the power of a conspirator, and then disarmed him with the force of his reproaches, shows that the king was also credited with exceptional moral stature. His byname, Ceann Mór, meaning 'Big Head', from which comes Canmore, is probably to be understood literally. However, the same sobriquet is also recorded in a twelfth-century source as being applied to his great-grandson Malcolm IV, prompting speculation that he suffered from Paget's disease. Malcolm III is not certainly referred to as Canmore before the thirteenth century, though contemporary accounts refer to his florid complexion and long neck. Malcolm was succeeded as king by his brother Donald III.

Although no significant innovator, Malcolm III, by his vigorous rule and his remarkable second marriage, left Scotland a very different kingdom from what it had been before 1050. It was more clearly defined territorially and more decisively within the orbit of an English kingdom conquered by the Normans, and was appreciably less oriented towards the Scandinavian north. Malcolm and Margaret may be said to have put Scotland on the European map.