William I (William the Lion, William Leo, William Dunkeld or William Canmore), (1142/1143 - December 4 1214) reigned as King of Scotland from 1165 to 1214. His reign was the longest in Scottish history. He became King following his brother Malcolm IV's death on 9 December 1165 and was crowned on 24 December 1165.

Traditionally, William founded Arbroath Abbey, the site of the later Declaration of Arbroath. He became known as "the Lion" because of his flag or standard, a red lion rampant on a yellow background. This went on to become the Royal standard of Scotland; the British Monarch when in Scotland still uses it today. The rampant lion also forms part of the Royal Standard of the United Kingdom.

William also arranged the Auld Alliance, the first treaty for mutual self-defence between nations. Scotland, France, and Norway subscribed to the treaty. Although Norway never took much part in it, it played a role in Franco-Scottish (and English) affairs until 1746.

William also inherited the title of Earl of Northumbria in 1152. However he had to give up this title to King Henry II of England in 1157. This caused trouble after William became king, since he spent a lot of effort trying to regain Northumbria.

William was a key rebel in the Revolt of 1173-1174 against Henry II. In 1174, during a raid in support of the revolt William was captured by Henry's troops and taken in chains to Northampton, and then transferred to Falaise in Normandy. Henry then sent an army to Scotland and occupied it. As ransom and to regain his kingdom, William had to acknowledge Henry as his feudal superior and agree to pay for the cost of the English army's occupation of Scotland by taxing the Scots. This he did by signing the Treaty of Falaise . He was then allowed to return to Scotland.

The Treaty of Falaise remained in force for the next fifteen years. At the end of that time the new English king, Richard the Lionheart, agreed to terminate it in return for 10,000 silver marks. Richard needed the money to take part in the Third Crusade.

William died in Stirling in 1214 and lies buried in Arbroath Abbey. His son, Alexander II, succeeded him as king.