The agency said Dr Tedros had led a "comprehensive reform effort " of Ethiopia's health system, creating health centres and jobs.
The candidates are Tedros Adhanom Ghebreyesus, a 52-year-old former government minister in Ethiopia; Sania Nishtar, a 54-year-old cardiologist and former government minister from Pakistan; and David Nabarro, 67, a physician and longtime United Nations official from Britain.
Tedros will take over on July 1, succeeding Margaret Chan, a Hong Kong native whose decade-long tenure was marred by the agency's fiercely criticised response to the Ebola epidemic in west Africa. He received 133 votes while Nabarro got 50, with two abstentions. The two others were David Nabarro from the United Kingdom, and Sania Nishtar from Pakistan, in a process that began before September 2016. Outgoing director-general Dr. Margaret Chan admitted that World Health Organization failed to respond quickly or forcefully enough to the West African Ebola outbreak in 2014.
Nabarro tweeted his congratulations to Tedros after the vote, writing, "I urge everyone to unite behind him & his vision".
Dr. Tedros' efforts to improve Ethiopia's health sector were crucial to doubling the percentage of Ethiopian women with access to contraception.
This is indeed the holy grail of global health, and Dr Tedros is now the man in charge of pressing governments to make it happen.
Addressing the World Health Assembly shortly before the vote, Dr Tedros promised to respond to future emergencies "rapidly and effectively".
"The campaign has ended, as you know, officially, but I think the work begins actually now".
"The United States is committed to helping advance reforms and cultivating greater global health security", he said.
Just as such countries need to expand their revenue base, World Health Organization should also widen its income sources, he argued.
"When it happens the world will turn to the WHO for guidance and for leadership".
Tedros takes over a World Health Organization that has struggled to live up to expectations.
The victor will succeed Dr. Margaret Chan, who's ending a 10-year tenure.
U.S. Secretary for Health and Human Services Tom Price, in a speech to the WHO's annual ministerial assembly on Monday, did not refer to U.S. contributions while voicing support for WHO.
The U.N. agency's chief has considerable power to set global medical priorities and declare health emergencies, such as outbreaks of the Zika or Ebola viruses.