The banners of former communist Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia and Slovenia were hoisted on Friday in the courtyard of the sprawling NATO complex in a Brussels suburb four days after they joined, raising membership to 26.
The three Baltic states are former Soviet republics whose incorporation into the Western alliance has deeply riled Moscow.
But Russian Foreign Minister Sergei Lavrov was due to meet his NATO colleagues later on Friday in what Western diplomats saw as a signal of acceptance.
"It's a historical moment. For 50 years we were occupied by the Russians. We've never been as safe as we are today," said Lithuanian warrant officer Algirdas Nakvosas.
NATO warplanes began air patrols over the Baltic states as soon as they acceded on Monday, despite complaints from Moscow.
Former Lithuanian president Vytautas Landsbergis, a hero of independence from the Soviet Union in 1991, told state radio: "People have been trying to tell us that the Cold War is over, that Russia is different. But there are many facts showing us that this is an illusion.
"I speak not of the Russian people, but in the minds of Russian leaders nothing is different from 10 or even 15 years ago. The Cold War against the Baltic States continues."
Bulgarian Foreign Minister Solomon Passy shed a tear as his country's flag was raised and the military band of Supreme Headquarters Allied Powers Europe played its national anthem.
Bulgaria, Romania and Slovakia once formed part of the Soviet-led Warsaw Pact, NATO's foe which collapsed in 1991. Slovenia once belonged to non-aligned but communist Yugoslavia.
In their talks, the ministers were to focus on slow delivery of pledges to expand security in Afghanistan.
They also planned to discuss the recent setback in Kosovo to their efforts to stabilise peace in the Balkans and announce steps to fight terrorism with Mediterranean partners, but avoid any deeper involvement in Iraq, diplomats said.