US President Donald Trump on Tuesday signed the US instrument of ratification for Montenegro's accession to NATO, the White House said in a statement.
Last month the Senate overwhelmingly backed the expansion of NATO to allow Montenegro to join the alliance, hoping to send a message that the United States will push back against Russian efforts to increase its influence in Europe.
The foreign ministers of NATO member states invited Montenegro to join the military alliance as its 29th member in December 2015.
NATO last expanded eight years ago with the joining of Albania and Croatia.
Russia, which has close economic ties with Montenegro, has repeatedly objected to the country joining NATO and has also said it would consider such a move a "provocation," and a threat to stability in the western Balkans.
Tensions between NATO and Russia have been rising since Moscow unilaterally annexed Crimea and has given support to separatist forces in eastern Ukraine.
Montenegrins divided on NATO
NATO’s invitation had also met opposition within Montenegro. Opposition figures in the country still hold NATO accountable for bombing Montenegro back in 1999 when it was part of the state of Serbia.
NATO staged a 79-day bombing campaign against Serbia and Montenegro to force Slobodan Milosevic’s forces to withdraw from Kosovo.
Montenegro was the last state to break off from Serbia, which constituted the heartland of what was once the Socialist Federal Republic of Yugoslavia.
In late 2016, Montenegrin authorities claimed to have thwarted an attempt by Serb and Russian nationalists to assassinate then-prime minister Milo Djukanovic and get an opposition alliance into power.
They allegedly coordinated the coup plot from Serbia and tracked Djukanovic's movements.
The suspects reportedly escaped to Russia.
Russia has denied involvement, but has actively supported local groups that oppose Montenegro, a traditional Slavic ally, from joining NATO.