Martti Oiva Kalevi Ahtisaari (born 23 June 1937) was the tenth President of Finland (1994–2000), 2008 Nobel Peace Prize laureate and United Nations diplomat and mediator, noted for his international peace work.

Ahtisaari was a UN Special Envoy at the Kosovo status process negotiations, aimed at resolving a long-running dispute in Kosovo, which declared its independence from Serbia in 2008. In October 2008, he was awarded the Nobel Peace Prize "for his important efforts, on several continents and over more than three decades, to resolve international conflicts". The Nobel statement said that Ahtisaari has played a prominent role in resolving many conflicts in Namibia, Indonesia, Kosovo and Iraq, among other areas.

Martti Ahtisaari was born in Viipuri, Finland (now Vyborg, Russia). His father, Oiva Ahtisaari (whose grandfather Julius Marenius Adolfsen had emigrated with his parents to Finland in 1872 from Tistedalen in southern Norway) took Finnish citizenship in 1929 and changed his surname from Adolfsen in 1937. The Continuation War (World War II) took Martti's father to the Eastern Front as a non-commissioned officer army mechanic, while his mother, Tyyne, moved to Kuopio with her son to escape immediate danger from the war. Kuopio was where Ahtisaari spent most of his childhood and first attended school "Kuopion Lyseo".

In 1952, Martti Ahtisaari moved to Oulu with his family to seek employment. There he continued his education in a well-known high school "Oulun Lyseo" (among its former students are two other presidents of Finland: Kaarlo Juho Ståhlberg and Kyösti Kallio) and graduated in 1952. He also joined the local YMCA. After completing his military service (Ahtisaari holds the rank of captain in the Finnish Army Reserve), he began to study through a distance-learning course at Oulu teachers' college. There he was able to live at home while attending the two-year course which enabled him to qualify as a primary-school teacher in 1959. Besides his native language, Finnish, Ahtisaari speaks Swedish, French, English, and German.

In 1960, he moved to Karachi, Pakistan, to lead the YMCA's physical education training establishment, where he became accustomed to a more international environment. As well as managing the students' home, Ahtisaari's job involved training teachers, which suited him well. He returned to Finland in 1963. He became active in non-governmental organizations (NGOs) responsible for aid to developing countries, and joined the international students' organization AIESEC, where he discovered new passions about diversity, and diplomacy. In 1965, he joined the Ministry for Foreign Affairs of Finland in its Bureau for International Development Aid, eventually becoming the assistant head of the department. In 1968, he married Eeva Irmeli Hyvärinen (1936– ). The couple have one son, Marko Ahtisaari, a noted musician and producer. Ahtisaari spent several years as a diplomatic representative from Finland. From 1977 to 1981, he served as United Nations Commissioner for Namibia, working to secure the independence of Namibia from the Republic of South Africa.

Following the death of a later UN Commissioner for Namibia, Bernt Carlsson, on Pan Am Flight 103 on December 21, 1988 – on the eve of the signing of the Tripartite Accord at UN headquarters – Ahtisaari was sent to Namibia in April 1989 as the UN Special Representative to head the United Nations Transition Assistance Group (UNTAG). Because of the illegal incursion of SWAPO troops from Angola, the South African appointed Administrator-General (AG), Louis Pienaar, sought Ahtisaari's agreement to the deployment of SADF troops to stabilize the situation. Ahtisaari took advice from British prime minister Margaret Thatcher, who was visiting the region at the time, and approved the SADF deployment. A period of intense fighting ensued when at least 375 SWAPO insurgents were killed. In July 1989, Glenys Kinnock and Tessa Blackstone of the British Council of Churches visited Namibia and reported: "There is a widespread feeling that too many concessions were made to South African personnel and preferences and that Martti Ahtisaari was not forceful enough in his dealings with the South Africans." Perhaps because of his reluctance to authorise this SADF deployment, Ahtisaari was alleged to have been targeted by the South African Civil Cooperation Bureau (CCB). According to a hearing in September 2000 of the South African Truth and Reconciliation Commission, two CCB operatives (Kobus le Roux and Ferdinand Barnard) were tasked not to kill Ahtisaari, but to give him "a good hiding". To carry out the assault, Barnard had planned to use the grip handle of a metal saw as a knuckleduster. In the event, Ahtisaari did not attend the meeting at the Keetmanshoop Hotel, where Le Roux and Barnard lay in wait for him, and thus Ahtisaari escaped injury. After the independence elections of 1989, Ahtisaari was appointed an honorary Namibian citizen. South Africa gave him the O R Tambo award for "his outstanding achievement as a diplomat and commitment to the cause of freedom in Africa and peace in the world".

Ahtisaari served as UN undersecretary general for administration and management from 1987 to 1991 causing mixed feelings inside the organization during an internal investigation of massive fraud. When Ahtisaari revealed in 1990 that he had secretly lengthened the grace period allowing UN officials to return misappropriated taxpayer money from the original three months to three years, the investigators were furious. The 340 officials found guilty of fraud were able to return money even after their crime had been proven. The harshest punishment was the firing of twenty corrupt officials.

Ahtisaari's presidential campaign in Finland began when he was still a member of the council dealing with Bosnia. Finland's ongoing recession caused established political figures to lose public support, and the presidential elections were now direct, instead of being conducted through an electoral college. In 1993, Ahtisaari accepted the candidacy of the Social Democratic Party. His politically untarnished image was a major factor in the election, as was his vision of Finland as an active participant in international affairs. Ahtisaari narrowly won over his second round opponent, Elisabeth Rehn of the Swedish People's Party. During the campaign, there were rumours spread by some political opponents of Ahtisaari that he had a drinking problem or that he had knowingly accepted a double salary from the Finnish Foreign Ministry and from the United Nations while trying to negotiate an end to the Bosnian War. Ahtisaari denied both allegations and no firm proof of them has emerged. During the three-week campaign between the two rounds of presidential elections, Ahtisaari was praised by his supporters for being more compassionate towards the many unemployed Finns than Rehn, who as Defence Minister had to officially support the Aho government's strict economic policies. A minor scandal arose during a town hall-style presidential debate in Lappeenranta, southeastern Finland, when an apparently born-again Christian woman in the audience asked Rehn what her relationship with Jesus was. Rehn replied that she had personally no proof that Jesus had been a historical person. Ahtisaari ducked a precise answer by stating that he trusted the Lutheran confession even on this issue.

His term as president began with a schism within the Centre Party government led by prime minister Esko Aho, who did not approve of Ahtisaari's being actively involved in foreign policy. There was also some controversy over Ahtisaari's speaking out on domestic issues such as unemployment. He travelled extensively in Finland and abroad, and was nicknamed "Matka-Mara" ("Travel-Mara," Mara being a common diminutive form of Martti). His monthly travels throughout the country and his meetings with ordinary citizens (the so-called maakuntamatkat or "provincial trips") nonetheless greatly enhanced his political popularity. Ahtisaari kept his campaign promise to visit one Finnish historical province every month during his presidency. He also donated some thousands of Finnish marks per month to the unemployed people's organizations, and a few thousand Finnish marks to the Christian social organization of the late lay preacher and social worker Veikko Hursti.

Although Ahtisaari favoured pluralism and religious tolerance, at least publicly, he and his wife apparently privately practise their Christian faith. Contrary to some of his predecessors and his successor as the Finnish President, Ahtisaari ended all of his New Year's speeches by wishing the Finnish people God's blessing.

In January 1998 Ahtisaari was criticized by some NGOs, politicians and notable cultural figures because he awarded medals of honour to the Forest Minister of Indonesia and to the main owner of the Indonesian RGM Company, a parent company of the April Company. The April Company was criticized by non-governmental organizations for destroying rain forests, and Indonesia itself was criticized heavily for human right violations, especially in East Timor. Ahtisaari's party chairman Erkki Tuomioja said that giving medals was questionable since he feared the act may tarnish the public image of Finnish human rights policy. Students of the arts had demonstrations in Helsinki against the decision to give medals.

President Ahtisaari supported Finland's entry into the European Union, and in a 1994 referendum, 57 percent of Finnish voters were in favour of EU membership. During Ahtisaari's term as president, Boris Yeltsin and Bill Clinton met in Helsinki. He also negotiated alongside Viktor Chernomyrdin with Slobodan Miloševic' to end the fighting in the Yugoslav province of Kosovo in 1999.

Often encountering resistance from the Finnish parliament, which preferred a more cautious foreign policy, as well as from within his own party, Ahtisaari did not seek re-election in 2000. He wanted the Social Democrats to re-nominate him for the presidency without opposition, but two opponents signed up for the party's presidential primary. Ahtisaari was the last "strong president", since the 2000 constitution slightly reduced the president's powers. He was succeeded by the foreign minister Tarja Halonen. The two disagree on many issues.

In Finnish politics, Ahtisaari has stressed how important it is for Finland to join NATO. Ahtisaari has argued that Finland should be a full member of NATO and the EU in order "to shrug off once and for all the burden of Finlandization". He believes politicians should file application and make Finland a member. He says that the way Finnish politicians avoid expressing their opinion is disturbing. He has noted that the so-called "NATO option" is an illusion, making an analogy to trying to obtain fire insurance when the fire has already started.

Since leaving office, Ahtisaari has held positions in various international organizations. Ahtisaari also founded the independent Crisis Management Initiative (CMI) with the goal of developing and sustaining peace in troubled areas. On December 1, 2000, Ahtisaari was awarded the J. William Fulbright Prize for International Understanding by the Fulbright Association in recognition of his work as peacemaker in some of the world's most troubled areas. In 2000–01, Ahtisaari and Cyril Ramaphosa inspected IRA weapons dumps for the Independent International Commission on Decommissioning, as part of the Northern Ireland peace process. In 2005, Ahtisaari successfully led peace negotiations between the Free Aceh Movement (GAM) and the Indonesian government through his non-governmental organization CMI. The negotiations ended on August 15, 2005 with a treaty on disarmament of GAM rebels, the dropping of GAM demands for an independent Aceh, and a withdrawal of Indonesian forces.

In November 2005, UN Secretary-General Kofi Annan appointed Ahtisaari as Special Envoy for the Kosovo status process which was to determine whether Kosovo, having been administered by the United Nations since 1999, should become independent or remain a province of Serbia. In early 2006, Ahtisaari opened the UN Office of the Special Envoy for Kosovo (UNOSEK) in Vienna, Austria, from where he conducted the Kosovo status negotiations. Those opposed to Ahtisaari's settlement proposal, which involved an internationally-monitored independence for Kosovo, sought to discredit him. Allegations made by Balkan media sources of corruption and improper conduct by Ahtisaari were described by US State Department spokesman Tom Casey as "spurious", adding that Ahtisaari's plan is the "best solution possible" and has the "full endorsement of the United States". The New York Times suggested that this criticism of Ahtisaari on the part of the Serbs had led to the "bogging down" of the Kosovo status talks. In November 2008, Serbian media reported Pierre Mirel, director of the EU enlargement commission's western Balkans division as saying: “The EU has accepted that the deployment of EULEX has to be approved by the United Nations Security Council, and that the mission has to be neutral and will not be related to the Ahtisaari plan,” Mirel said, following his meeting with Serbia’s vice-president Bozidar Djelic. In July 2007, however, when the EU, Russia and the United States agreed to find a new format for the talks, Ahtisaari announced that he regarded his mission as over. Since neither the UN nor the troika had asked him to continue mediations in the face of Russia's persistent refusal to support independence for Kosovo, he said he would nonetheless be willing to take on "a role as consultant", if requested. After a period of uncertainty and mounting tension, Kosovo unilaterally declared its independence from Serbia in February 2008.

As a former head of state, Ahtisaari is a member of the Club of Madrid. In 2008 Ahtisaari was awarded an honorary degree by University College, London. That same year he received the 2007 UNESCO Félix Houphouët-Boigny Peace Prize, for "his lifetime contribution to world peace". On October 10, 2008 Ahtisaari was announced as that year's recipient of the Nobel Peace Prize. The award includes a medal, a personal diploma, and 10 million Swedish kronor ($1.4 million) in prize money. Ahtisaari received the prize on December 10, 2008 at Oslo City Hall in Norway. Ahtisaari twice worked to find a solution in Kosovo – first in 1999 and again between 2005 and 2007. He also worked with others this year to find a peaceful solution to the problems in Iraq, the Committee said. According to the Committee, Ahtisaari and his group, Crisis Management Initiative (CMI), also contributed to resolving other conflicts in Northern Ireland, Central Asia, and the Horn of Africa. Ahtisaari invited Prime Minister Matti Vanhanen, Foreign Affairs Minister Alexander Stubb and others to his Nobel event, but not President Halonen.