The Einstein–Szilard letter was a letter written by Leo Szilard and signed by Albert Einstein on August 2, 1939, that was sent to President of the United States Franklin D. Roosevelt. Written by Szilard in consultation with fellow Hungarian physicists Edward Teller and Eugene Wigner, the letter warned that Germany might develop atomic bombs and suggested that the United States should start its own nuclear program. It prompted action by Roosevelt, which eventually resulted in the Manhattan Project, the development of the first atomic bombs, and the use of these bombs on the cities of Hiroshima and Nagasaki.

Leo Szilard was a Hungarian born physicist and inventor. He conceived the nuclear chain reaction in 1933, patented the idea in 1936, and in late 1939 wrote the letter for Albert Einstein's signature that resulted in the Manhattan Project that built the atomic bomb. According to György Marx, he was one of the Hungarian scientists known as The Martians.