Bohr was born in Copenhagen, Denmark, on 7 October 1885, the second of three children of Christian Bohr, a professor of physiology at the University of Copenhagen, and Ellen Bohr (née Adler), who was the daughter of David B. Adler from the wealthy Danish Jewish Adler banking family. He had an elder sister, Jenny, and a younger brother Harald. Jenny became a teacher, while Harald became a mathematician and footballer who played for the Danish national team at the 1908 Summer Olympics in London. Niels was a passionate footballer as well, and the two brothers played several matches for the Copenhagen-based Akademisk Boldklub (Academic Football Club), with Niels as goalkeeper.