Harold Sidney Harmsworth, 1st Viscount Rothermere (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 London Daily Mail and Daily Mirror. He was a pioneer of popular journalism. He was also the elder brother of the Liberal politician Cecil Harmsworth, 1st Baron Harmsworth.
Rothermere's descendants continue to control the Daily Mail and General Trust.
Many in England were caught off-guard by Rothermere's impassioned endorsement of the Hungarian cause; it was rumoured that the press baron had been convinced to support it by the charms of a Hungarian seductress (she turned out to be Austrian). Rothermere's son Esmond was received with royal pomp during a visit to Budapest, and some political actors in Hungary later went so far as to inquire about Rothermere's interest in being placed on the Hungarian throne. Rothermere later insisted he did not invite these overtures, and that he quietly deflected them. His private correspondence indicates otherwise. He did purchase estates in Hungary in case Britain should fall to a Soviet invasion. There is a memorial to Rothermere in Budapest.
In 1934, a Mercury-engined version of the Bristol Type 135 cabin monoplane was ordered by Rothermere for his own use as part of a campaign to popularise commercial aviation. First flying in 1935, the aircraft caused great interest in Air Ministry circles because its top speed of 307 mph was higher than that of any Royal Air Force fighter in service. Lord Rothermere presented the aircraft (named "Britain First") to the nation for evaluation as a bomber and in early 1936 the modified design was taken into production as the Blenheim Mk.I
Secret British government papers released in 2005 show that Rothermere wrote to Adolf Hitler congratulating him for the annexation of Czechoslovakia in 1938, and encouraged him to march into Romania. The MI5 papers also show that Rothermere paid a retainer of £5,000 per year to Stephanie Hohenlohe-Waldenburg-Schillingsfürst, a glamorous Austrian princess and German spy, intending that she should bring him closer to Hitler's inner circle. She was known as "London's leading Nazi hostess". The secret services had been monitoring her since she came to Britain in the 1920s and regarded her as "an extremely dangerous person". As World War II loomed, Rothermere stopped the payments and their relationship deteriorated into threats and lawsuits.