"Tomorrow we will welcome Finland as the 31st member," Stoltenberg told reporters on the eve of a historic meeting of NATO foreign ministers in Brussels.
Objections from Turkey and Hungary held up Helsinki's bid for months, and are still blocking Stockholm, before the parliament in Ankara cleared the final obstacle for Finland with a vote last week.
Completing the ratification in well under a year still makes this the fastest membership process in the alliance's recent history.
Now all that is left are the last highly choreographed formalities at NATO headquarters in Brussels.
Finland's foreign minister will hand over the formal accession papers to US Secretary of State Antony Blinken, the keeper of NATO's founding treaty, and the country's blue-and-white flag will be raised alongside those of its new allies.
Finland's President Sauli Niinisto will speak at the event, Helsinki said.
Russian President Vladimir Putin's all-out invasion of Ukraine last year upended European security and pushed Finland - and its neighbour Sweden - to drop decades of non-alignment and seek to join NATO's protective umbrella.
"President Putin went to war against Ukraine with a clear aim to get less NATO," Stoltenberg said.
"He's getting the exact opposite."