Irina Bokova, a former foreign minister for Bulgaria, has been selected to lead the United Nations Agency for Education, Culture and Science, beating the leading candidate for the post.
If her appointment is endorsed in October by Unesco's 193-member assembly, she will take over from Japan's Koichiro Matsuura to become the first woman to lead the UN body.
Bokova won the fifth and final round of voting on Tuesday against Egypt's Farouk Hosni by 31 votes to 27, diplomats said.
Bokova joined Bulgaria's foreign ministry UN and disarmament department in 1976, becoming the country's foreign minister for a brief period in 1996-1997.
She has witnessed Bulgaria's transformation from a nation within the Eastern bloc to a European Union member, and will be the first person from the region to head Unesco.
Nine candidates were in the running for the senior job at Unesco when the body's council began voting last Thursday, but seven dropped out one-by-one.
Delegates had been split over the two remaining candidates, with Hosni, Egypt's minister of culture, facing accusations that he is an anti-Semite.
Hosni caused concern among some observers after he was quoted last year as saying he would burn Hebrew-language books.
Hosni has insisted his comments were part of an angry exchange in parliament with politicians from the Muslim Brotherhood and were taken out of context.
But his critics said that the book-burning comments made him unfit for the role.
Hosni's detractors included Elie Wiesel, an Auschwitz death camp survivor and Nobel laureate, who said the global cmmunity would be shamed if he was appointed.
Hosni has been associated with media censorship in his home country.
He also sparked anger among Muslim groups there when he said the hijab was a "step backward" for Egyptian women.
Supporters of Hosni said his election would have sent a positive signal from the West to the Muslim world, pointing to Hosni's efforts to rebuild a crumbling old synagogue in Cairo, Egypt's capital.