The Norwegian Nobel Committee said Ahtisaari, 71, "has figured prominently in endeavors to resolve several serious and long-lasting conflicts," mentioning his roles in Namibia, Kosovo and Iraq.
He wins a medal, a diploma and $1.4 million.
"Ahtisaari is an outstanding international mediator," said Ole Danbolt Mjoes, chairman of the Norwegian Nobel Committee.
"Through his untiring efforts and good results, he has shown what role mediation of various kinds can play in the resolution of international conflicts."
Ahtisaari and his organization, Crisis Management Initiative, were also involved in resolving other conflicts in Northern Ireland, Central Asia, and the Horn of Africa, the committee said.
Last year, the prize was shared by ex-U.S. vice president Al Gore and the Intergovernment Panel on Climate Change.
Ahtisaari will receive the prize in Oslo on December 10, the anniversary of the death of Swedish industrialist Alfred Nobel, who established the prizes in his will.
The peace prize is one of five Nobel prizes awarded annually. The other Nobel awards will be handed out by Sweden's King Carl XVI Gustaf in Stockholm the same day.
This year's winners were chosen by a secretive five-member committee from 197 nominations, including 33 organizations.
