"No one better than he could win the Nobel Peace Prize," former U.N. Secretary-General Kofi Annan said of the award, according to wire reports. "He is the only man I know who has made peace on three continents -- Africa, Asia and Europe -- and I always found him ready to answer the call to make this world a better place."
Among his accomplishments, Ahtisaari brokered peace between Indonesia and rebels in Aceh province in 2005, ending 30 years of fighting in the region. Until March of last year as U.N. envoy, he led Serb-Albanian talks on Kosovo.
He was also the designer of a European Union-backed plan for Kosovo's independence from Serbia, which guaranteed Serb minority rights and was implemented peacefully after the wars that tore apart Yugoslavia in the 1990s.
Ahtisaari, 71, also is credited with playing a key role in establishing independence for the African nation of Namibia.
"These efforts have contributed to a more peaceful world and to 'fraternity between nations' in Alfred Nobel's spirit," the Nobel committee said in announcing the prize.
"He has also made constructive contributions to the resolution of conflicts in Northern Ireland, in Central Asia and on the Horn of Africa," the citation said.
Indonesian President Susilo Bambang Yudhoyono was pleased to hear Ahtisaari had won the prize, a spokesman said, according to the Associated Press.
"We have known him to be a man of honor, a man of integrity and a man who not only has full devotion to the cause of peace but also has the rare talents to help make it to practical priority on the ground," the spokesman said.
Ahtisaari encountered bouts of tough opposition in his years as a peace negotiator. Serbia forcefully rejected his attempts to forge a compromise on Kosovo, which declared independence in February. Ahtisaari's blueprint forms the foundation of Kosovo's constitution.
Serbia considers Kosovo its historic and religious heartland and has long maintained that it should remain a Serbian province.
"He is a professional diplomat and a very skilful negotiator," Serbia's former Foreign Minister Zivadin Jovanovic told Reuters. "However, in Serbia he represents a symbol of Kosovo's independence and he will be remembered as an architect of Kosovo's secession."
By selecting Ahtisaari for the peace prize, the Nobel committee returned to the historical focus of the award after changing tracks last year and giving the honor to climate activist and former Vice President Al Gore and the U.N. panel on climate change.
Ahtisaari spent the early part of his career as a primary school teacher and later joined Finland's Foreign Ministry in 1965. He spent 20 years abroad, first as ambassador to Tanzania and then to the U.N. in New York.
Ahtisaari is the first peace broker to win the prize since former President Jimmy Carter in 2002 and the first European in a decade. He is also the first Finnish peace laureate.
"We have been waiting for a long time for this. He really deserved it. He has worked for a long time on solving problems of different groups. That's what he is really good at," Helsinki resident Leni Tornqvist told Reuters.