"The Russian people have a right to express their views peacefully without fear of retribution of any kind, and certainly not with chemical agents," wrote US National Security Council spokesman John Ullyot.
NATO chief Jens Stoltenberg condemned the "shocking" use of a military-grade nerve agent which, he said, made "a full and transparent" investigation by Russia even more urgent.
Italy's foreign ministry and Canadian foreign minister Francois-Philippe Champagne also condemned Navalny's poisoning.
The nerve agent, whose name means "newbie" or "newcomer" in Russian, can be deployed in an ultra-fine powder, liquid, or vapor.
It was famously used against ex-double agent Sergei Skripal in Britain in 2018, an assassination attempt that the West believes was ordered by the Kremlin despite Russia's denials.
Navalny fell ill after boarding a plane in Siberia last month, with aides saying they suspect he drank a cup of spiked tea at the airport.
He was initially treated in a local Siberian hospital, where doctors said they were unable to find any toxic substances in his blood, before he was flown to Berlin for specialized treatment on August 22.
The Kremlin, which has previously questioned the credibility of German doctors, said Russia was ready to cooperate fully.