Lagarde, the first woman to hold the post, was the only candidate nominated. Her new term starts on 5 July.
The 60-year-old was named the IMF's 11th managing director in 2011, and has led the multinational lending agency through the European debt crisis and a period of global economic stagnation she labelled the "New Mediocre".
In a statement, Aleksei Mozhin, dean of the IMF's executive board, said board members were impressed with Lagarde's "strong and wise leadership".
Before joining the IMF, she was an international lawyer. She also has served as France's minister of trade, agriculture and finance.
During the financial crisis of 2008, she led a council of European Union finance and economic ministers.