STORY: The signing away from the cameras at NATO headquarters follows a deal with Turkey at last week's NATO summit in Madrid, where Ankara lifted its veto on the Nordic membership bids following assurances that both countries would do more to fight terrorism.
"This is truly an historic moment," NATO Secretary-General Jens Stoltenberg said alongside the foreign ministers of the two countries.
The protocol means Helsinki and Stockholm can participate in NATO meetings and have greater access to intelligence but will not be protected by the NATO defence clause - that an attack on one ally is an attack against all - until ratification. That is likely to take up to a year.
It was at an allied summit in Madrid in 1997 that Hungary, Poland and the Czech Republic were invited to join, in the first of several waves of NATO's eastern expansion - seen as an achievement for the West but which has angered Russia.
Moscow has repeatedly warned Sweden and Finland against joining NATO. On March 12, the Russian foreign ministry said "there will be serious military and political consequences".