Secretary-General Jens Stoltenberg said members were united in condemning the “horrific” attack.
He added that there was “proof beyond a doubt” that a Novichok nerve agent was used against Mr Navalny.
But Russia has rejected the diagnosis made by doctors in Germany, where he is being treated.
Speaking after an emergency NATO meeting, Stoltenberg said the Kremlin “must fully cooperate with the Organization for the Prohibition of Chemical Weapons on an impartial international investigation”.
“We also call on Russia to provide full disclosure of the Novichok program to the OPCW,” he added.
The Soviet-era nerve agent was also used to poison ex-spy Sergei Skripal and his daughter in the UK in 2018. Britain accused Russian military intelligence of carrying out the attack and , as part of a coordinated effort, 20 countries were expelled. more than 100 Russian diplomats and spies. Russia has denied any involvement.
However, Mr Stoltenberg pointed out that the poisoning of Mr Navalny, which took place in Russia and not in a NATO member state, was different from that of the Skripals.
But several senior Russian deputies have brushed aside NATO’s latest demands.
“Until experts have confirmed or denied the use of chemicals subject to the Chemical Weapons Convention, calls for the OPCW’s involvement seem, in my opinion, politicized,” said Konstantin Kosachev of the Council of the Russian Federation.
Mr. Navalny, an anti-corruption activist, has long been the most prominent face of Russia’s opposition to President Vladimir Putin.
He fell ill last month while on a flight from Siberia to Moscow. The plane made an emergency landing in Omsk and Russian officials were persuaded to let it be flown to Germany two days later.
The Kremlin says it has not seen German data on Mr Navalny’s condition and therefore does not accept the diagnosis of poisoning.
Since the incident, the EU has demanded a “transparent” investigation from the Russian government. The United States National Security Council has also pledged to “work with allies and the international community to hold those in Russia to account.”