Sándor Simonyi-Semadam (23 March 1864 – 4 June 1946) was a Hungarian politician who served as prime minister for a few months in 1920.
He signed the Treaty of Trianon after World War I on 4 June 1920.
By this treaty Hungary lost a considerable part of its territory.
Simonyi was a member of the Hungarian-Nippon Society, a society for creating cultural links between Japan and Hungary.
After his term as prime minister he was involved in the financial sector.
He was a board member of various banks .
His daughter, Erzsébet Simonyi-Semadam had a son, Erno Simonyi, a prominent lawyer, and Károly Simonyi, a nuclear physicist and university professor (father of Charles Simonyi) was raised as an adopted child.
On 4 June 1946, the twenty-sixth anniversary of the signing of the Treaty of Trianon, Simonyi-Semadam died at his home in Budapest.