Harold Sydney Harmsworth, 1st Viscount Rothermere (1868-1940) was a British newspaper proprietor of Irish extraction. He was the younger brother of another leading newspaper proprietor Alfred Harmsworth, 1st Viscount Northcliffe.

Harmsworth founded the Glasgow Daily Record , and the Sunday Pictorial , but his greatest success came with the Daily Mirror, which had a circulation of three million by 1922. His elder brother died without an heir in that year, and he acquired control of his newspaper the Daily Mail.

Harmsworth was Air Minister for a time during World War I, and was raised to the peerage as Viscount Rothermere.

He has long been known as vociferous supporter of appeasement. Papers released by the British National Archives in 2005 document that Rothermere congratulated Hitler on his annexation of Czechoslovakia in 1938 and urged him to march into Romania. The papers also show that he had recruited Stephanie Hohenlohe-Waldenburg-Schillingfurst, a glamorous German spy and princess, to get closer to Hitler's inner circle. The princess, whom he paid £5,000 per year, was known to British intelligence and regarded as "exremely dangerous". In the build-up to the war, the two fell out, threatening and suing each other.

Rothermere's descendants continue to control the Daily Mail and General Trust plc.