Sir Denis Le Marchant, 1st Baronet (3 July 1795 – 30 October 1874), was a British barrister, civil servant, writer and Whig politician.
The member of an old Guernsey family, Le Marchant was born at Newcastle-upon-Tyne, the second son of Major-General John Le Marchant and his wife Mary née Carey, eldest daughter of John Carey, of Guernsey. His father was killed at the Battle of Salamanca in 1812 while his elder brother Carey also died in the Peninsular War. His younger brother John Le Marchant became a distinguished colonial administrator. He was educated at High Wycombe Royal Grammar School,Eton and Trinity College, Cambridge, and was called to the Bar, Lincoln's Inn, in 1823.
Le Marchant appeared for the petitioner in the Gardner Peerage Claim and published Proceedings of the House of Lords in the Gardner Peerage Claim in 1828. In 1830 he was appointed principal secretary to Lord Brougham, the Lord Chancellor, on the recommendation of Brougham's brother, William Brougham, who Le Marchant had befriended at Cambridge. He gained distinction during the debates over the Great Reform Act where he was of great assistance to government ministers. In 1834 he was appointed Clerk of the Crown in Chancery and in the same year he edited a successful pamphlet, The Reform Ministry and the Reform Parliament, to which his close friend Lord Althorp, and also Lord Stanley, Lord Palmerston, and Sir James Graham were contributors. From 1836 to 1841 he was secretary to the Board of Trade and also served briefly as joint secretary to the Treasury in 1841. The latter year he was created a Baronet, of Chobham Place in the County of Surrey.