Sir William Wallace (Scottish Gaelic: Uilleam Uallas, pronounced [ˈɯʎam ˈuəl̪ˠəs̪]; Norman French: William le Waleys; c. 1270 – 23 August 1305) was a Scottish knight who became one of the main leaders during the First War of Scottish Independence.

Along with Andrew Moray, Wallace defeated an English army at the Battle of Stirling Bridge in September 1297. He was appointed Guardian of Scotland and served until his defeat at the Battle of Falkirk in July 1298. In August 1305, Wallace was captured in Robroyston, near Glasgow, and handed over to King Edward I of England, who had him hanged, drawn and quartered for high treason and crimes against English civilians.

Since his death, Wallace has obtained a legendary status beyond his homeland. He is the protagonist of Blind Harry's 15th-century epic poem The Wallace and the subject of literary works by Jane Porter and Sir Walter Scott, and of the Academy Award-winning film Braveheart.

William Wallace was a member of the lesser nobility, but little is definitely known of his family history or even his parentage. William's own seal, found on a letter sent to the Hanse city of Lübeck in 1297, gives his father's name as Alan Wallace. This Alan Wallace may be the same as the one listed in the 1296 Ragman Rolls as a crown tenant in Ayrshire, but there is no additional confirmation. Others have speculated this Alan held Ellerslie, near Kilmarnock, Ayrshire, and if true, the estate could be a possible birthplace for William, though there is no record of Wallaces holding the estate in the mid-13th century. Blind Harry's late-15th-century poem offers an alternate father for William, a Sir Malcolm of Elderslie, in Renfrewshire, and has similarly given rise to a possible birthplace for William. There is no contemporary evidence linking him with either location, although both areas had connections with the wider Wallace family. Records show early members of the family as holding estates at Riccarton, Tarbolton, Auchincruive in Kyle and Stenton in East Lothian. They were vassals of James Stewart, 5th High Steward of Scotland, as their lands fell within his territory. It has been claimed that Wallace's brothers Malcolm and John are known from other sources, but there is a lack of verifiable evidence for John's relationship with William.

The origins of the Wallace surname and its association with southwest Scotland are also far from certain, other than the name's being derived from the Old English wylisc (pronounced 'wullish'), meaning 'foreigner' or 'Welshman'. It is possible that all the Wallaces in the Clyde area were medieval immigrants from Wales, but, as the term was also used for the Cumbric-speaking Strathclyde kingdom of the Celtic Britons, it seems equally likely that the surname refers to people who were seen as being "Welsh" due to their Cumbric language.

When Wallace was growing up, King Alexander III ruled Scotland. His reign had seen a period of peace and economic stability. On 19 March 1286, however, Alexander died after falling from his horse. The heir to the throne was Alexander's granddaughter, Margaret, Maid of Norway. As she was still a child and in Norway, the Scottish lords set up a government of guardians. Margaret fell ill on the voyage to Scotland and died in Orkney in late September 1290. The lack of a clear heir led to a period known as the "Great Cause", with a total of thirteen contenders laying claim to the throne. The most credible claims were John Balliol and Robert Bruce, grandfather of the future monarch, Robert the Bruce.

With Scotland threatening to descend into civil war, King Edward I of England was invited in by the Scottish nobility to arbitrate. Before the process could begin, he insisted that all of the contenders recognise him as Lord Paramount of Scotland. In early November 1292, at a great feudal court held in the castle at Berwick-upon-Tweed, judgment was given in favour of John Balliol having the strongest claim in law based on being senior in genealogical primogeniture even though not in proximity of blood.

Edward proceeded to take steps to progressively undermine John's authority, treating Scotland as a feudal vassal state, demanding homage be paid towards himself and military support in his war against France—even summoning King John Balliol to stand before the English court as a common plaintiff. The Scots soon tired of their deeply compromised king, and the direction of affairs was allegedly taken out of his hands by the leading men of the kingdom, who appointed a Council of Twelve—in practice, a new panel of Guardians—at Stirling in July 1295. They went on to conclude a treaty of mutual assistance with France—known in later years as the Auld Alliance.