NATO's changing Administration. The muscle behind UN.

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Jens Stoltenberg impressed national leaders in his time as Norway's prime minister. Associated Press

Jens Stoltenberg, former prime minister of Norway, was selected as the next leader of the North Atlantic Treaty Organization, putting him at the helm as the alliance faces a historic challenge.

Mr. Stoltenberg, who succeeds Anders Fogh Rasmussen, will take office Oct. 1, becoming secretary-general as NATO faces an array of new challenges in the wake of Russia's incursion into Ukraine.

There had been some discussion in the run-up to the decision about appointing a secretary-general from one of NATO's newer members in Central or Eastern Europe, given that many have belonged to the alliance for at least a decade, diplomats said. But Mr. Stoltenberg's credentials won him broad support. The only other major candidate was Italy's Franco Frattini, a former defense minister, officials said.

Mr. Stoltenberg is well-known to NATO leaders from his nine years as premier of oil-rich Norway. He became known on the global stage in 2011, taking a very public role in helping his nation recover from a terror attack staged by Anders Behring Breivik, who claimed the lives of 77 people.

"The Ukraine-Russia crisis shows need for continued strong [and] determined leadership," Mr. Rasmussen, a former Danish prime minister who had led NATO since mid-2009, tweeted after the announcement. "I've known Jens Stoltenberg for many years [and] know he's the right man to build on NATO's record of strength and success."

The Obama administration also hailed Mr. Stoltenberg's appointment. "As prime minister, he built Norway's military capabilities and actively contributed to NATO operations and political dialogue," the White House said in a statement. "We are confident he is the best person to ensure the continued strength and unity of the NATO Alliance."

NATO, created to contain the former Soviet Union, had been struggling to define its purpose in recent months, especially with the mission to Afghanistan winding down. But the Ukraine crisis has pushed the 28-member organization into the spotlight, given its role as a collective defense group with some members bordering and in proximity to Russia.

Jens Stoltenberg

Date of birth: March 16, 1959

Son of Thorvald Stoltenberg and Karin Stoltenberg

Married to Ingrid Schulerud, two children

Norway Labor Party leader, 2002-2014

Norway Minister of Trade and Energy, 1993-1996

Norway Minister of Finance, 1996-1997

Norway Prime Minister, 2000-2001, 2005-2013

U.N. Special Envoy on Climate Change, 2013-

Speaking at a news conference in Oslo, Mr. Stoltenberg said that NATO had "once again proven its relevance in a critical situation in Europe.

"I believe very strongly that what we've seen in Ukraine just reminds us of how important NATO is and that the idea of NATO collective defense is just becoming even more important when we see how Russia is using force to change borders in Europe," he said.

Still, the alliance's immediate response to Russia's aggressiveness has been limited to actions such as ramping up air patrols over the Baltic region. Leaders from NATO countries are holding a summit in Wales in September to discuss the alliance's future, and President Barack Obama is expected to underscore the need for solidarity in response to events in Ukraine.

Mr. Stoltenberg is known for his advocacy of causes such as fighting global climate change rather than as a military leader. But he has a reputation as a strong NATO supporter during his time at Norway's helm and is seen as a steady and pragmatic politician.

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"It takes a lot more strength to be patient than to be angry," Mr. Stoltenberg said in a 2011 book, referring to the challenges of keeping together a three-party coalition government.

Mr. Stoltenberg was known in his early years as a critic of NATO, but some analysts attributed that to his youthful participation in politics.

"In Norway you have a very strong culture of youth parties," said Erik Dale, a Norwegian and online editor for Bruegel, a Brussels-based think tank. "Sure, he could have been seen as anti-American or anti-NATO in his youth, but that was 30 years ago."

The appointment of Mr. Stoltenberg, a leader of Norway's center-left Labor Party for 12 years, comes after his government's swift decision to contribute six F-16 aircraft to the 2011 NATO bombing campaign in Libya, as well as the participation of Norwegian troops in Afghanistan.

Norway is also playing a significant role in the mission to dismantle Syria's chemical weapons program, which isn't a NATO operation, but nonetheless a major international security initiative.

Mr. Stoltenberg's experience in dealing with Russia was cited by diplomats as a last-minute sweetener to his candidacy. He negotiated a 2010 deal with Moscow on the Barents Sea, ending a decadeslong border dispute and opening the area for oil and gas exploration.

But like other European leaders, Mr. Stoltenberg has firmly criticized Russia for its incursion into Ukraine.

Mr. Stoltenberg, 55, the son of diplomat and former minister Thorvald Stoltenberg, became a government minister at 34 and prime minister at 41.

A terrorist attack in downtown Oslo and the summer camp of Labor's youth association on July 22, 2011, which left 77 people dead and destroyed several government buildings, heavily affected his tenure.

"The answer to violence is even more democracy. Even more humanity. But never naiveté," Mr. Stoltenberg said after the attack.

But while he was hailed for leading his nation through the tragedy, it may have contributed to his 2013 elections loss to right-leaning Erna Solberg. Reports issued by Norway following the attack criticized Mr. Stoltenberg's government for failing to protect its citizens.

Write to Naftali Bendavid at naftali.bendavid@wsj.com and Kjetil Malkenes Hovland at kjetilmalkenes.hovland@wsj.com