Mrs. Okonjo-Iweala will become the first woman and the first African to lead the WTO since its inception in 1995.
She takes over the institution, with its budget of $220m and staff strength of 650 people come March 1.
Fellow contender for the office, South Korea’s Yoo Myung-hee, had announced her withdrawal in a televised address on Friday.
An economist and former finance minister of Nigeria, Mrs. Okonjo-Iweala had gotten support from major WTO members, such as the European Union, China, Japan and Australia.
However, the United States, under the Trump administration, had favored Yoo, delaying the decision-making process since the selection of a new leader requires the agreement of its members.
The Geneva-based body has had a vacuum in its leadership since former DG Roberto Azevêdo stepped down earlier than his tenure required last August.
Dr. Okonjo-Iweala spent 25 years at the World Bank as a development economist, rising to the position of managing director.
She was also the chair of the board of Gavi, helping in the global distribution of coronavirus vaccines.