He has been in the post since the 1 October 2014 and will now stay until the 30 September 2022.
So what do we know about Jen Stoltenberg?
He was born in Oslo on 16 March 1959.
Most of his childhood was spent abroad with his diplomat father, mother, and two sisters.
He went to the University of Oslo where he studied Economics, before graduating in 1987.
Mr Stoltenberg's long political career started in 1990 when he was made Norway's State Secretary at the Ministry of the Environment.
He went on to become a Minister of Industry and Energy and then a Minister of Finance, before becoming the Prime Minister of Norway in 2000.
Mr Stoltenberg held the post of Prime Minister for a year and then he went on to lead the Norwegian Labour Party.
Four years later, he returned to the Prime Ministerial post in 2005, staying until 2013.
According to NATO, while Mr Stoltenberg was Prime Minister, Norway's defence spending increased steadily, resulting in the country being one of the Allies with the highest per capita defence expenditure today.
The organisation also said Mr Stoltenberg was instrumental in transforming the Norwegian Armed Forces, through a strong focus on "deployable high-end capabilities".
In 2014, he became Secretary General of NATO and was due to complete his tenure in 2020, before it was extended until 2022.
Elisabeth Braw, Associate Fellow at defence think-tank the Royal United Services Institute, was asked why NATO has taken this decision.
Transatlantic ties remain one of his priorities.
"My main task is to maintain that unity," he told reporters last year, referring to relations between North America and Europe.
Mr Stoltenberg has also struck a more conciliatory tone towards Moscow than his Danish predecessor, Anders Fogh Rasmussen.
“I believe in dialogue with Russia but I am also here to ensure security in Europe,” he said.
NATO has gained new relevance as the West confronts a resurgent Russia, state-sponsored computer hackers, security threats on its borders, and militant attacks on European cities.
Mr Stoltenberg has also thrown his weight behind diplomatic efforts to contain the North Korea's nuclear weapons, visiting US allies in Japan and South Korea last October.
On social media, Mr Stoltenberg said he was "honoured" that his term has been extended, and said: "Together we will continue to adapt & modernise our Alliance to keep it fit for the future."