William FitzWilliam, 1st Earl of Southampton

William FitzWilliam, 1st Earl of Southampton, KG (c. 1490 in Aldwark, North Yorkshire – 15 October 1542 in Newcastle upon Tyne), English courtier, was the third son of Sir Thomas FitzWilliam of Aldwark and Lady Lucy Neville (daughter of the Marquess of Montagu).

His father died while FitzWilliam was in his infancy, and his mother remarried Sir Anthony Browne senior so that William was half-brother to Sir Anthony Browne. Probably as a result of this connection, he was chosen as a companion for Henry, Prince of Wales (later King Henry VIII) and brought up alongside him. After King Henry's coronation in 1509, he was made a Gentleman Usher and King's Cupbearer, and gradually rose at Court. He began his military career at sea, serving under the Marquess of Dorset in 1512 and Sir Edward Howard in the disastrous second attack on Brest. Unlike his commander, he escaped the debacle, but was badly injured by a crossbow bolt. He had recovered sufficiently to accompany the King into France as an Esquire of the Body, and was knighted on 25 September 1513, the day after the capture of Tournai. In November he married Mabel Clifford, daughter of the Lord Clifford, but the marriage would prove childless.

FitzWilliam achieved distinction as naval commander, as diplomat and as government minister. Much of his time as Vice-Admiral (1513–1525) and Admiral was spent keeping the Channel free from pirates, and he gained praise from Wolsey for his initiative in actions against the French in the war of 1522/3. His time as ambassador at the French court in 1521/2 attracted favourable notice from Wolsey and showed his suitability for higher office. Later missions failed: Henry's obsession with the divorce gave his ambassadors little scope for initiative. He became Treasurer of the Household in 1525, a post which gave him an ex officio seat on the evolving Privy Council. He was appointed Captain of Guines in 1524 and maintained a connection with Calais for the rest of his life, being largely responsible for the Calais Act of 1536; he also played a significant part in defusing religious unrest in Calais in the later 1530s. He was a capable Lord Privy Seal from 1540 to his death in 1542, but he failed to address serious structural faults at the Duchy of Lancaster and the Admiralty administration, probably because with several offices he was overworked - a serious fault in the Tudor system. As regional magnate he eliminated factional strife in Surrey.

He acted as "enforcer" for Henry in the fall of Anne Boleyn and Catherine Howard, the Pilgrimage of Grace and the Exeter Conspiracy. In 1539, as Admiral, he conveyed Anne of Cleves from Calais, and on first meeting her wrote letters in her praise to Henry, 'considering it was then no time to dispraise her, ... the matter being so far passed.'