The appointment comes at a critical time as the crisis over Ukraine has suddenly made the 28-nation alliance a more relevant security force in Europe.
NATO Secretary General Anders Fogh Rasmussen will step down after a NATO summit in Wales later this year.
"Warm congratulations" Fogh Rasmussen said in a Twitter message Friday to his fellow Scandinavian politician.
The past weeks had seen a flurry of diplomacy as member states sought to push their candidates into NATO's top political job.
Norwegian Prime Minister Erna Solberg, who replaced Stoltenberg as prime minister last year, said NATO was getting "a strong and unifying secretary general."
"It is a huge and responsible task Stoltenberg has received today," Solberg said in a statement.
A two-time Norwegian prime minister, Stoltenberg became a recognizable face on the international scene with his dignified response to the twin terror attacks that killed 77 people in July 2011.
Stoltenberg's pledge at the memorial service to combat the atrocity with "more democracy, more openness, and more humanity" helped salve the country's wounds.
His coalition suffered a year later when an independent inquest into the bomb and gun attacks by right-wing fanatic Anders Breivik found a litany of failures by police and security services that might have disrupted or even prevented the slaughter. By September 2013, Stoltenberg's coalition government had been ousted by a combination of conservatives and populists as the Norway tilted right.
Jan M. Olsen contributed from Copenhagen
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