Montenegro and NATO have signed a protocol on the country's accession to the alliance, paving the way for the Balkan country to become the 29th member of the military alliance.
Montenegrin Prime Minister Milo Djukanovic attended the signing ceremony at NATO headquarters in Brussels on May 19.
NATO Secretary-General Jens Stoltenberg said it was the "beginning of a new secure chapter" in the former Yugoslav republic's history.
Montenegro will participate in the alliance's meetings as an observer until all NATO nations ratify its accession protocol.
Djukanovic said Montenegro expects to join the alliance by the middle of 2017, becoming the third nation from the Western Balkans to join NATO after Albania and Croatia.
Russia has threatened to respond to Montenegro's membership, amid concerns in Moscow about NATO's presence moving increasingly closer to its border as a threat.