Norway's ex-PM Jens Stoltenberg has been appointed Nato's next secretary general, replacing Denmark's Anders Fogh Rasmussen
He will take over as leader of the 28-nation military alliance in October, when Mr Rasmussen steps down after a Nato summit in Wales.
Mr Rasmussen has headed Nato for the past five years.
Mr Stoltenberg was prime minister of Norway at the time of Anders Breivik's bomb and gun attacks in 2011.
Breivik killed 77 people when he targeted government buildings in Oslo and young people and officials at a youth camp run by Mr Stoltenberg's Labour party.
He served as the country's prime minister twice, for 18 months in 2000-01 and again from 2005 to 2013.
Mr Rasmussen said on Twitter that Mr Stoltenberg was "the right man to build on Nato's record of strength and success".
He added that the Ukraine crisis demonstrated the need for Nato to show strong and determined leadership.
Mr Rasmussen expressed concern on Friday about the "huge military build-up" by Russian forces on Ukraine's borders.
UK Prime Minister David Cameron said Norway's ex-prime minister would "bring a wealth of experience to this role at the helm of the transatlantic alliance and he will take over during a crucial year in the history of the organisation".
As well as tackling the crisis in Ukraine, Mr Cameron said he would lead the withdrawal of Nato-led forces in Afghanistan at the end of the year.
In its statement, Nato said: "Mr Stoltenberg will assume his functions as secretary general as from 1 October 2014, when Mr Fogh Rasmussen's term expires after five years and two months at the helm of the alliance."