Philip Sidney, 3rd Earl of Leicester (10 January 1619 – 6 March 1698) was an English politician who sat in the House of Commons at various times between 1640 and 1659 and inherited the peerage of Earl of Leicester in 1677. He supported the Parliamentary cause in the English Civil War. During the Wars of the Three Kingdoms, he was known as Viscount Lisle or (Lord Lisle) a subsidiary title of the Earls of Leicester.
Sidney was the son of Robert Sidney, 2nd Earl of Leicester and his wife Dorothy Percy, daughter of Henry Percy, 9th Earl of Northumberland. In April 1640, he was elected Member of Parliament for Yarmouth, Isle of Wight in the Short Parliament. He was elected MP for both Yarmouth and St Ives for the Long Parliament in November 1640, and chose to sit for Yarmouth. He was Colonel of a Regiment of Horse in Ireland in 1641.
Lord Lisle supported the Parliamentary cause in the civil war and was Lord Lieutenant and Commander-in-Chief of Ireland from 1646 to 1647. He survived Pride's Purge in 1648 to sit in the Rump Parliament and was a Councillor of State from 1648 to 1650. He was appointed a judge for the trial of King Charles I but declined to act. He was President of the Council from 1651 to 1652. He was Councillor of State and Councillor to the Lord Protector in 1653. Also in 1653, he was elected MP for Kent in the Barebones Parliament. In 1654 he was elected MP for Isle of Wight, a constituency that only existed in the First Protectorate Parliament. He was appointed to Cromwell's "House of Lords" in 1658 under the designation "Lord Viscount Lisle". In 1659 he was returned to the House of Commons for the Restored Rump parliament.
On the restoration of King Charles II in 1660 Lord Lisle received a pardon. In 1677 he inherited the Earldom on the death of his father.
Lord Lisle married Lady Catherine Cecil, daughter of William Cecil, 2nd Earl of Salisbury and Lady Catherine Howard in 1645. Their children were Dorothy and Robert; the latter succeeded to his father's earldom.
Two of Lord Lisle's brothers supported parliament in the Civil War. Algernon Sydney was a Parliamentarian "martyr", but Henry Sydney, 1st Earl of Romney did not follow the cause to the same treasonous extremes.