North Macedonia officially joined NATO on Friday, becoming the 30th member of the alliance after resolving a long-running name row with Greece.
The US State Department said the Balkan republic had submitted its "instrument of accession" to the NATO treaty in Washington - completing the final formal step in the membership process.
The move caps a successful week of diplomacy for North Macedonia, after EU governments agreed - after several delays - that they would start membership talks with Skopje.
"North Macedonia's accession to NATO today represents the culmination of many years of effort by the government and people of North Macedonia to join the North Atlantic Alliance," US Secretary of State Mike Pompeo said in a statement.
"North Macedonia's NATO membership will support greater integration, democratic reform, trade, security, and stability across the region."
NATO countries signed North Macedonia's accession protocol in February last year, after which all 29 national parliaments ratified the country's membership, ending with Spain.
The way to NATO and EU membership was opened by a deal struck with Greece in January last year to change the name of the former Yugoslav republic to the Republic of North Macedonia, ending ended one of the world's longest diplomatic disputes.