A Harvard-educated New York lawyer from a family of Hungarian Jews, Ferencz secured convictions against 22 Nazi death squad commanders following World War II. He turned 103 on March 11.
His son confirmed his father's death to NBC News on Saturday.
US Media reported on Saturday that Ben Ferencz died on Friday at an assisted living facility in Boynton Beach, Florida.
He is survived by his son, three daughters and three grandchildren, the newspaper said. His wife passed away in 2019.
Ferencz was 27 at the time of the Nuremberg Trials (conducted between November 1945 and October 1946) and later played a major role in securing reparations payments for Holocaust survivors and creating the International Criminal Court at The Hague, according to NBC News.