"Northern Macedonia is now a member of the NATO family, a family of 30 countries and about a billion people, a family that is confident that we are stronger and safer together, whatever challenges we face," NATO Secretary-General Jens Stoltenberg said in a statement.
Stoltenberg will chair a ceremonial event at NATO headquarters in Brussels on Monday, during which he will raise the flag of Northern Macedonia next to the other 29 flags.
Reuters news agency quoted US Secretary of State Mike Pompeo as saying: "The accession of North Macedonia to NATO today marks the culmination of years of efforts by the government of Northern Macedonia and its people to join NATO."
"The membership of North Macedonia in NATO will promote integration, democratic reform, trade, security and stability in the region," he added.
The way to join NATO and the European Union was opened to Northern Macedonia after it reached an agreement with Greece in January last year, to change its name to the Northern Republic of Macedonia, ending one of the longest diplomatic disputes in the world.