"I didn't plan to be here," the 65-year-old Stoltenberg told reporters as he took possession of his new office at the finance ministry.
When his close friend Prime Minister Jonas Gahr Støre offered him the job last week, "the only correct answer was yes", he added.
Speaking of the risk of a trade war, he stressed the importance of "doing everything we can to protect ourselves against the creation of trade barriers against Norway".
Stoltenberg was Norwegian prime minister between 2000 and 2001 and again from 2005 to 2013 before serving as NATO secretary general between 2014 and 2024, when he developed good ties with Donald Trump during the US president's first term.
His appointment as finance minister comes after the collapse of Norway's coalition government last week, when the junior Centre party quit the government.
The eurosceptic party disagreed with Labour's intention to implement EU energy directives.
The move left the Labour Party alone in government with a very weak minority in parliament, and meant that Støre had to find eight new ministers to fill empty positions in his cabinet.
An economist by training, Stoltenberg has previously served as minister of industry in the 1990s and then as finance minister, in the wealthy Scandinavian country with vast oil and gas reserves and hydro power.
"Give us a chance to show that we can lead Norway safely with modern social democratic values," Støre told reporters at a press conference.
'Vitamin injection'
Political observers said that Stoltenberg's unexpected return to domestic politics was expected to give Labour a boost seven months ahead of legislative elections, given his popularity among Norwegians.
"A strong vitamin injection," said TV2 political commentator Aslak Eriksrud.
Stoltenberg's ties with Trump could also prove useful at a time when Norway, which is not a member of the European Union, fears that its open and trade-dependent economy could suffer if a customs war breaks out between the United States and Europe.
Trump has threatened to impose tariffs on the EU, and Brussels has warned that such a move would be met with countermeasures.
Although it has access to the single market through its membership of the European Economic Area, Norway is not part of the customs union.
After leaving NATO, Stoltenberg was named chairman of the Munich Security Conference (MSC), viewed as the "Davos of defence" that gathers the geopolitical elite every year.
In a statement from the MSC on Tuesday, Stoltenberg said he would return to that position when his tenure in government was over.
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