Announcing the prize in Stockholm, the chair of the Swedish Academy, Sara Danius, called her writing "a monument to courage and suffering in our time".
Alexievich called the award, presented to a living writer and worth 8m kronor (£691,000), "a great personal joy."
Previous winners include literary heavyweights Rudyard Kipling and Ernest Hemingway.
French novelist Patrick Modiano won in 2014. It has been half a century since a writer working primarily in non-fiction won the Nobel - and Alexievich is the first journalist to win the award.
"It's not an award for me but for our culture, for our small country, which has been caught in a grinder throughout history," she said at a press conference on Thursday at a local newspaper's offices in Minsk, Belarus.
Her best-known works in English translation include Voices from Chernobyl, an oral history of the 1986 nuclear catastrophe; and Zinky Boys, a collection of first-hand accounts from the Soviet-Afghan war. The title refers to the zinc coffins in which the dead came home.
The book caused controversy and outrage when it was first published in Russia, where reviewers called it a "slanderous piece of fantasy" and part of a "hysterical chorus of malign attacks".
Alexievich has also been critical of her home country's government, leading to a period of persecution - in which her telephone was bugged and she was banned from making public appearances.
She spent 10 years in exile from 2000, living in Italy, France, Germany and Sweden, among other places, before moving back to Minsk.
"I love the Russian world, but the kind, humane Russian world," she added, talking of the country under President Vladimir Putin.
"I do not love Beria, Stalin, Putin... how low they let Russia sink."
The author was born in 1948 in the Ukrainian town of Ivano-Frankivsk, then known as Stanislav,to a Belarusian father and Ukrainian mother.
The family moved to Belarus after her father completed his military service, and Alexievich studied journalism at the University of Minsk between 1967 and 1972.
After graduation, she worked as a journalist for several years before publishing her first book, War's Unwomanly Face, in 1985.
Based on interviews with hundreds of women who participated in the World War Two, it set a template for her future works, constructing narratives from witnesses to some the world's most devastating events.
On her personal website, Alexievich explains her pursuit of journalism: "I chose a genre where human voices speak for themselves."
Alexievich has previously won the Swedish PEN prize for her "courage and dignity as a writer".
Ms Danius said the author had spent nearly 40 years studying the people of the former Soviet Union, but that her work was not only about history but "something eternal, a glimpse of eternity".
"By means of her extraordinary method - a carefully composed collage of human voices - Alexievich deepens our comprehension of an entire era," the Swedish Academy added.
Alexievich was the bookmakers' favourite to win 2015 Nobel award, according to Ladbrokes.
She beat other hot favourites Japanese novelist Haruki Murakami and Kenyan novelist Ngugi Wa Thiong'o.
She is the 14th woman to win the Nobel Prize for Literature in its history.
A total of 112 individuals have won it between 1901 and 2015. The prize was suspended several times during the first and second world wars.
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