Sir William Stanley (c. 1435^[1] – 10 February 1495) was an English soldier and the younger brother of Thomas Stanley, 1st Earl of Derby. Stanley fought with his troops in several battles of the Wars of the Roses.

A noble who originally supported the Yorkist faction, he was a celebrated military commander. He fought on the Yorkist side at Blore Heath in 1459. In 1465 he was granted the lands and titles of the dispossessed Lancastrian Cliffords. After the Battle of Tewkesbury in 1471, it was he who captured Queen Margaret of Anjou, who led the Lancastrian faction and was made a Knight Banneret by the king. In 1483 he was made Chief Justice of North Wales. After Richard III came to the throne he was awarded more land in North Wales for his loyal services.^[2]

However, by 1485 he had decided to change sides and support the LacastriaN Henry Tudor's bid for the throne. Stanley is best known for his action at the Battle of Bosworth Field, where he decisively attacked the Yorkists under Richard III, helping to secure Henry VII's victory. In gratitude for his timely intervention, the new king bestowed many favours on him, including the new post of Lord Chamberlain.

However, in 1495 Stanley was convicted of treason and executed for his support of the pretender Perkin Warbeck. He readily admitted to the crime despite circumstantial evidence as he thought that through a full confession he would escape execution, given Henry's merciful track record.^{[citation needed]} Indeed the King might have granted this, partly through mercy and partly to avoid upsetting Thomas, Earl of Derby. Since the King feared that by doing this he would be putting himself in danger by encouraging others to undertake a similar act of folly William was condemned to death and a few days later beheaded.