Knight, 51, is of British nationality. She replaces Duncan MacLaren, who leaves the office after eight years. She will assist newly elected president, Cardinal Oscar Rodríguez Maradiaga.
The election came during the organization's 18th general assembly, held this week in the Vatican.
Knight said: "Caritas is a treasure. I look forward with great honor to the next four years at Caritas Internationalis. The work we do here has a lasting impact on the lives of the poor, and I feel humbled to be given this responsibility by my brothers and sisters in the Caritas family.
"Our priorities will be to put humanitarian emergencies, integral human development, and building sustainable peace at the heart of the work of the confederation, not as diverse pieces of work, but as all part of the same program that will help the poor transform their own lives."
Knight was the international director for Caritas England and Wales.