"Peace on earth depends on our ability to secure our living environment," the head of the Norwegian Nobel Committee, Ole Danbolt Mjoes, said on Friday in announcing the winner. He praised her "contribution to sustainable development, democracy and peace."
"Maathai stands at the front of the fight to promote ecologically viable social, economic and cultural development in Kenya and in Africa," she said.
Maathai won the prize, worth 10 million Swedish crowns ($1.36 million), from a record field of 194 candidates. Named after Swedish philanthropist Alfred Nobel, the inventor of dynamite, the prize is handed out in Oslo on 10 December.
Maathai is founder of the Kenya-based Green Belt Movement, comprised mainly of women, which says it has planted about 30 million trees across Africa.
Born in 1940, Maathai says that tree plantings slow desertification, preserve forest habitats for wildlife and provide a source of fuel, building materials and food for future generations to help combat poverty.
"This is the biggest
surprise in my entire life. When we plant new trees we plant the seeds of peace"
"I am absolutely overwhelmed," she told Norway's NRK television after confirmation of the award. "This is the biggest surprise in my entire life. When we plant new trees we plant the seeds of peace."
Maathai is the first African woman to win the peace prize and the 12th woman peace laureate since the first award was made in 1901. The last African laureate was UN Secretary General Kofi Annan, of Ghana, in 2001.
The 2003 prize also went to a woman, Iranian human rights lawyer Shirin Ebadi. The current Nobel Committee, appointed by the Norwegian parliament early in 2003, comprises three women and two men.
New types of activists
Geir Lundestad, the head of the Norwegian Nobel Institute, said in 2001 that the award might shift in its second century to honour new types of activists such as environmentalists, rock stars, perhaps even journalists.
Iranian rights lawyer Shirin Ebadi
won the Nobel prize in 2003
The prize was a surprise. The UN's International Atomic Energy Agency and its head, Muhammad al-Baradai, had been widely tipped to win by peace experts.
The Kenyan government said it was very pleased the award had gone to Maathai. "This is a great testament to the work she has been doing for many years. We are very happy," said Kenyan government spokesman Alfred Mutua.
"It sets an example that if you put your energy into the right places you are eventually recognised and that it leads to a better world."