Sadako Ogata, 92, a former senior UN official who was the first and only woman to serve as the UN High Commissioner for Refugees, has died.
Ogata is best known for wearing a helmet and a bullet-proof vest during her field trips.
"Ogata died on October 22," Japanese government officials said, but announced her death on Tuesday.
Japanese Prime Minister Shinzo Abe said: "It has always been sympathetic to those who suffer."
Her tenure in office during the 1990s coincided with the genocide in Rwanda and the ethnic conflict in the former Yugoslavia.
Chief Cabinet Secretary Yoshihide Suga said: "The first ranks on humanitarian issues such as poverty, refugees and conflicts have shown top leadership."
"We would like to express our deepest respect and pray for her soul."
The UN High Commissioner for Refugees, Filippo Grandi, praised Ogata for her work.