William Keith, 7th Earl Marischal (1610 - 1670 or 1671) was a Scottish nobleman and Covenanter. He was the eldest son of William Keith, 6th Earl Marischal. He joined Montrose and twice seized Aberdeen in 1639, including a march with Montrose and 9000 men along the Causey Mounth past Muchalls Castle and through the Portlethen Moss to attack via the Bridge of Dee.
He was appointed a Lord of the Articles after the pacification of Berwick-upon-Tweed, and again seized Aberdeen and enforced signatures of the covenant in 1640. The 7th Earl Marischal was appointed a Privy Councillor in 1641.
He attended covenanting committees in the north but remained inactive in 1643-4. He subsequently refused to give up fugitives to Montrose, and was besieged at Dunnottar Castle in 1645. He then joined Hamilton's expedition into England in 1648 and entertained Charles II at Dunnottar in 1650. He was arrested and imprisoned in the Tower of London until the Restoration, when he was appointed Keeper of the Privy Seal of Scotland.
The Earl Marischal married, in 1637, Elizabeth Seton (1621–1650) daughter of George Seton, 3rd Earl of Winton by his spouse Anne, daughter of Francis Hay, 9th Earl of Erroll. They had the following issue:
- William, Lord Keith, died young.
- Mary, married firstly to Sir James Hope of Hopetoun in 1657, and secondly to Sir Archibald Murray, 3rd Baronet of Blackbarony.
- Elizabeth, married in 1658, Robert Arbuthnot, 2nd Viscount Arbuthnot
- Jean, married in 1669, George Ogilvy, 3rd Lord Banff