William Douglas Cullen, Baron Cullen of Whitekirk, PC 18. november 1935), is Scotland's most senior judge. He was appointed as Lord Justice General and Lord President of the Court of Session in 2001. He is a cross bench member of the House of Lords and was created a life peer in 2003, as Baron Cullen of Whitekirk, of Whitekirk in East Lothian, having been previously known as Lord Cullen by courtesy of his membership of the Court of Session.

William Douglas Cullen was educated at the High School of Dundee and the universities of St Andrews and Edinburgh. He was admitted to the Faculty of Advocates in 1960 and was Standing Junior Counsel to Her Majesty's Customs and Excise from 1970 to 1973. He was appointed Queen's Counsel in 1973 and served as an Advocate Depute from 1978 to 1981.

Lord Cullen was a Chairman of Medical Appeal Tribunals from 1977 until his appointment as a judge in 1986. From 1988 to 1990 he conducted the Public Inquiry into the Piper Alpha disaster and in 1996 he chaired the Public Inquiry into the shootings at Dunblane Primary School. In October 1999 he was appointed to chair the Ladbroke Grove Rail Inquiry. He was Lord Justice Clerk and President of the Second Division of the Inner House from 1997 to 2001.

On 15 July 2005 Lord Cullen announced his intention to retire in November 2005. On 24 November the Scottish Executive announced that his successor would be Arthur Hamilton, Lord Hamilton, a member of the Inner House of the Court of Session.