The incident happened on Saturday, Biden's office said in a statement, with the 78-year-old Democrat visiting an orthopedist on Sunday for x-rays and a CT scan.
Doctors initially thought the former vice president had merely sprained his ankle, but ordered an additional scan of the injury.
A "follow-up CT scan confirmed hairline (small) fractures," Biden's personal physician Kevin O'Connor said in a separate statement distributed by Biden's office. "He will likely require a walking boot for several weeks."
After defeating President Donald Trump in the Nov. 3 presidential election, Biden will be the oldest person ever to assume the office when he enters the White House on Jan. 20. His health is likely to be watched closely by allies and opponents alike.Trump, who has not conceded defeat in the election and is contesting the results in court, wished Biden a speedy recovery. "Get well soon!" Trump said in a Tweet.