Bacall was born Betty Joan Perske on September 16, 1924, in The Bronx, New York,the only child of Natalie (née Weinstein; 1901–1977), a secretary who later legally changed her surname to Bacall, and William Perske (1889–1982), who worked in sales. Both her parents were Jewish. According to Bacall, her mother emigrated from Iași, the Kingdom of Romania, through Ellis Island, and her father was born in New Jersey to parents who were born in Valozhyn, a significant center of Jewish life in present-day Belarus, then in the Russian Empire.
Soon after her birth, Bacall's family moved to Brooklyn's Ocean Parkway.
Her parents divorced when she was five; she later took the Romanian form of her mother's last name, Bacall. She no longer saw her father and formed a very close bond with her mother, who remarried Lee Goldberg and came to live in California after Bacall became a movie star.
Through her father, she was a relative of Shimon Peres (born Szymon Perski), the eighth Prime Minister of Israel and ninth President of Israel.