James Moncreiff, 1st Baron Moncreiff FRSE (29 November 1811 – 27 April 1895) was a Scottish lawyer and politician.
The son of Sir James Wellwood Moncreiff, 9th Baronet, a Scottish judge, and Ann, daughter of George Robertson, R. N., he was educated at Edinburgh University, and was admitted to the Faculty of Advocates in 1833. He was appointed Solicitor General for Scotland in 1850, and Lord Advocate from 1851 to 1852, from 1852 to 1858, from 1859 to 1866 and from 1868 to 1869. He was Dean of the Faculty of Advocates from 1858 to 1869. He was appointed Lord Justice Clerk from 1869 to 1888.
Moncreiff was appointed a Privy Counsellor in 1869. He was Rector of the University of Glasgow from 1868 to 1871, and held the degrees of LLD from both Edinburgh and Glasgow universities.
Moncreiff was Member of Parliament for Leith Burghs from 1851 to 1859, for Edinburgh from 1859 to 1868 and for Glasgow and Aberdeen Universities in 1868. During a long career in parliament Moncreiff guided the passing of over a hundred acts of parliament, and his name is associated with the reform of legal procedure and mercantile law. As lord advocate he was engaged as public prosecutor in important cases, notably the trials of Madeline Smith, Wielobycki, and the directors of the Western bank. In 1856, he defended the Scotsman in the libel action raised by Duncan McLaren, one of the members for the city of Edinburgh.