Harold Sidney Harmsworth, 1st Viscount Rothermere, (26 April 1868 – 26 November 1940) was a leading 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.