Finland will officially become Nato's 31st member on Tuesday, the Finnish President's office announced on Monday.
It follows Turkey’s approval of Helsinki’s membership bid last Thursday — the last hurdle to its entrance to the alliance.
Nato Secretary General Jens Stoltenberg hailed the decision.
“We will raise the Finnish flag for the first time here at Nato headquarters. It will be a good day for Finland's security, for Nordic security and for Nato as a whole,” he said in Brussels.
The flag-raising ceremony on Tuesday afternoon will come after Finland's Foreign Minister Pekka Haavisto hands over the formal accession papers to US Secretary of State Antony Blinken, the keeper of Nato's founding treaty.
The news comes a day after a Finnish general election in which Prime Minister Sanna Marin conceded defeat.
However, Nato accession is broadly supported by Finnish parties and President Sauli Niinisto - who was not up for election - has led the final stage of negotiations with Turkey.
After decades of non-alignment, Finland and Sweden applied to join Nato last year, following Russia's invasion of Ukraine.
While most members supported the bids and accepted their applications in June, Turkey and Hungary blocked the ratification process required from all Nato members.
Completing the ratification in well under a year still makes this the fastest membership process in the alliance's recent history.
Mr Stoltenberg said Finland's membership would double the length of Nato's land border with Russia.
This went against Russian President Vladimir Putin's aim to weaken the alliance, he said.
"Putin went to war against Ukraine with the clear aim to get less Nato," said Mr Stoltenberg. "He's getting the exact opposite."
In response, Russian Deputy Foreign Minister Alexander Grushko said Russia would strengthen its military capacity in its western and north-western regions, state-owned news agency RIA reported.
Finland's accession process has been smoother than Sweden's, which is still negotiating with Turkey.
Turkey accuses Stockholm of failing to crack down on Kurdish groups in Sweden, which Ankara considers to be terrorists.
Last month, Turkey said Helsinki had given it assurances that it would take measures against such groups based in Finland.
"We are still moving very quickly in this accession process also for Sweden," said Mr Stoltenberg.
"We shouldn't give the impression that Sweden is left alone."
But Nato's secretary general also said he recognised Turkey's "legitimate security concerns".
"When Sweden and Turkey are working closely together in fighting terrorism, that helps Turkey in its fight against the PKK," a Kurdish militant group labelled by Turkey and the EU as terrorist, he said.
Mr Stoltenberg then seemed to link organised crime in Sweden with the PKK.
"It also helps Sweden in addressing some of the organised crime that Sweden is seeing taking place in its own streets," he said.
"We know that there is a close link between many of these terrorist organisations and these activities and organised crime, drug trafficking, in a country like Sweden, so I strongly believe that we have a common interest in addressing these issues."
