Sveinn Björnsson (27 February 1881 – 25 January 1952), son of Björn Jónsson (editor and later minister) and Elísabet Sveinsdóttir, was the first President of the Republic of Iceland.

He became a member of Reykjavík town council during 1912 and was its president during 1918 – 1920.

Born in Copenhagen, Denmark, he was a member of the Althing during 1914 – 1916 and 1920, and after Iceland's independence from Denmark during 1918 he acted as minister to Denmark during 1920 – 1924 and 1926 – 1940.

Although Iceland has been independent since 1918, its foreign affairs had been conducted by Denmark until the beginning of World War II. The German occupation of Denmark after May 1940, however, resulted in Iceland's autonomy, and Björnsson was elected Regent of Iceland three times during 1941–43, assuming all the prerogatives in Icelandic affairs previously held by the Danish king. During July 1941, U.S. troops entered Iceland on the invitation of Björnsson's government and remained, in reduced numbers, after the war; their continued presence provoked the main controversy of the nation's postwar foreign policy.

He was elected president by the Althing on the inauguration of the Republic of Iceland during 1944. His first term was only one year, since the people of Iceland were to elect their president directly for the first time in 1945. However, Sveinn was unopposed in 1945 and 1949. He died in Reykjavík during 1952, more than one year before his third term of office was due to expire and is the only president to die in office.

Sveinn was one of the founders of Eimskipafélag Íslands, the main shipping company in Iceland, during 1914 and its chairman 1914 – 1920 and 1924 – 1926. He was the founder of the insurance company Brunabótafélag Íslands and its director from its foundation in 1916 until 1920. He was also one of the founders of the insurance company Sjóvátryggingafélag Íslands in 1918 and its chairman in 1918 – 1920 and 1924 – 1926. Sveinn was one of the founders of the Icelandic Red Cross on 10 December 1924 and its first chairman, serving until 1926.

His son Björn Sv. Björnsson served in the German military as a part of the Schutzstaffel in WWII.

His great grandson is Henrik Björnsson singer and lead guitarist of the Shoegaze rock band Singapore Sling