Reginald Baliol Brett, 2nd Viscount Esher, PC (30 June 1852 - 22 January 1930) was an historian and Liberal politician in the United Kingdom.

Brett was the son of William Baliol Brett, 1st Viscount Esher. He began his political career in 1880, as MP for Penrhyn . However, five years later, he elected to withdraw from public politics in favour of a behind the scenes role. Upon his father's death on 24 May 1899, he earned the hereditary peerage, Viscount Esher.

In 1901, Lord Esher became deputy governor (and later governor) of Windsor Castle, and remained close to the royal family until his death. During this period, he helped edit Queen Victoria's papers, publishing a work called Correspondence of Queen Victoria (1907). Behind the scenes, he influenced many of the pre-World War I reforms carried out by the Liberal governments of Henry Campbell-Bannerman and Herbert Henry Asquith, and was a supporter of the United Kingdom-France entente. He was offered many public offices, including the viceroyalty of India and the secretaryship for war, but declined, accepting instead an appointment to the Privy Council in 1922.

Lord Esher was also a historian; besides the aforementioned work, he also published works on King Edward VII and Lord Kitchener.

Lord Esher's daughter, Sylvia, became the last Ranee of Sarawak on May 24, 1917, following the proclamation of her husband Charles Anthoni Johnson Brooke as Rajah.