Harold Sidney Harmsworth, 1st Viscount Rothermere, Bt. (26 April 1868 – 26 November 1940) was a highly successful British newspaper proprietor, owner of Associated Newspapers Ltd. He is known in particular, with his brother Alfred Harmsworth, the later Viscount Northcliffe, for the development of the Daily Mail and Daily Mirror. He was a pioneer of popular journalism.

During the 1930s, he was known to be a supporter of Nazi Germany, purportedly having become convinced that the National Socialist Party would help restore the German monarchy. He cultivated contacts to promote British support for Germany.

Background

Harmsworth was the son of Alfred Harmsworth, a barrister, and the brother of Alfred Harmsworth, 1st Viscount Northcliffe, Cecil Harmsworth, 1st Baron Harmsworth, Sir Leicester Harmsworth, 1st Baronet, and Sir Hildebrand Harmsworth, 1st Baronet.

Harmsworth was educated at St Marylebone Grammar School, which he left to become a clerk for the Board of Trade. In 1888 he joined his elder brother Alfred's newspaper company, and in 1894 he and his brother purchased the Evening News for £25,000.