The "barbaric" act - using a neurotoxin that is classed as a chemical weapon - could only have been carried out by Vladimir Putin's government, they said on Saturday.
The poisoning "highly likely" resulted in Mr Navalny's death in 2024.
Moscow dismisses allegations - latest
Sky News understands it is likely the toxin was manufactured in a laboratory rather than actually taken from the frogs.
It is not clear how the frog poison - called epibatidine - was allegedly administered to the dissident, who had been in a penal colony in Siberia when he died almost exactly two years ago.
Indigenous tribes in South America are said to use the toxin in blow darts or blowguns when they hunt.
The poison - described as "one of the deadliest on earth" - is 200 times stronger than morphine. It causes paralysis, breathing difficulties and death.
Yulia Navalnaya, the Russian dissident's widow, appeared at a press conference on the sidelines of a security conference in Munich to announce the discovery, flanked by the foreign ministers of the UK, Germany, Sweden and the Netherlands.
The four countries and France have been working together to establish how Mr Navalny, 47, died.
They plan to submit their findings to the international chemical weapons watchdog, the Organisation for the Prohibition of Chemical Weapons (OPCW).
"It is hard for me to find the right words," Mrs Navalnya said in English, looking visibly upset.
She had been at the Munich Security Conference when the news of her husband's death broke on 16 February 2024.
"It was the most horrible day in my life. I came to the stage and I said my husband, Alexei Navalny, was poisoned. What could else happen with Putin's number one enemy in a Russian prison?
"But now I understand and I know it is not just words. It is scientific proof."
Russian authorities have previously claimed the dissident's death was not suspicious but had been caused as a result of "combined diseases", including an irregular heartbeat.
But Yvette Cooper, the British foreign secretary, said the UK and its allies had been working "with fierce determination" to establish what really happened.
It is understood that British scientists at Porton Down played a key role in the discovery of the frog poison scheme.
It was not immediately clear how they were able to obtain samples from Mr Navalny's body to test for the toxin, though Ms Navalnaya has previously said they were smuggled out of Russia.
The opposition leader was buried in the suburbs of Moscow in March 2024.
"As a result of the work of the UK, Sweden and other partners, we have confirmed that a deadly toxin was found in Alexei Navalny's body," Ms Cooper told journalists,
"And that toxin has been identified as a toxin that is found in Ecuadorian Dart Frogs."
She said: "Only the Russian government had the means, the motive and the opportunity to use that toxin against Alexei Navalny in prison and that is why we are here today to shine a spotlight on the Kremlin's barbaric attempt to silence Alexei Navalny's voice.
"To show that the Russian government has contempt for its citizens and the willingness to use this deadly toxin."
The top British diplomat also quoted Mr Navalny himself about the need to hold the Russian government to account.
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"He said: 'We must do what they fear, tell the truth, spread the truth that is the most powerful weapon'."
Ms Navalnaya shared details about the effect of being poisoned dart frog toxin.
"My husband, Alexei Navalny, was poisoned with epibatidine - a neurotoxin, one of the deadliest poisons on earth," she said in a post of social media.
"In nature, this poison can be found on the skin of the Ecuadorian dart frog. It causes paralysis, respiratory arrest, and a painful death."
Sweden's foreign minister, Maria Malmer Stenergard, explained why it was necessary to understand the true cause of Mr Navalny's death.
"This is extremely important in order to be able to hold Russia accountable for what they have been doing and to continue to shine a light on their continuous lies," he said.
"We will now move forward with this information to the OPCW... This is yet another way to increase the pressure on Russia."
The Kremlin dismissed the allegation, with foreign ministry spokesperson Maria Zakharova telling the TASS news agency it was part of a disinformation campaign "aimed at diverting attention from the pressing problems of the West".
The Russian Embassy in the UK went further, calling it a "ridiculous circus performance" aimed at stoking "waning anti-Russian sentiment", adding that "if there's no pretext, they [media outlets] laboriously invent one.
"This isn't a quest for justice, but a mockery of the dead. Even after the death of a Russian citizen, London and European capitals cannot give him peace, which speaks volumes about the instigators of this campaign."
President Putin's government has already been accused of previously trying to kill Mr Navalny in 2020 using a Novichok nerve agent - the same chemical weapon that Russia is believed to have deployed on the streets of Salisbury in a failed attempt to kill a former Russian double agent, Sergei Skripal, and his daughter in 2018.
Mr Navalny survived the original poisoning and recuperated in Germany before returning home to Russia, where he was arrested and jailed.
The extraordinary announcement on Saturday seemed deliberately timed to grab global headlines in much the same way as the dissident's actual death did.
The news that Mr Navalny, 47, had died in prison in Russia broke on the opening day of the same Munich Security Conference of global leaders almost exactly two years ago.
Naming the person or government allegedly responsible for such a political assassination - if the claim of poisoning is substantiated - is a form of information weapon.
The intent is to make sure perpetrators cannot hide in the shadows and could face serious consequences such as sanctions or even criminal convictions.
The thinking - which might well be wildly optimistic - is that greater scrutiny could make a hostile state think twice before sanctioning plots to kill political opponents or other enemies.
It is doubtful that the efforts by Yvette Cooper, the British foreign secretary, and her German, Swedish and Danish counterparts, who made the poisoning announcement, along with Mr Navalny's widow, will deter the Kremlin from such allegedly ruthless action in the future.
However, their words are at the very least evidence of a growing resolve amongst NATO allies to stand up to what they regard as attempts by President Vladimir Putin to attack their countries and societies in the grey zone - under the threshold of conventional war. This also includes acts of sabotage, cyber hacks and disinformation spread online.
Speaking to a group of journalists about the case, Ms Cooper spelt out clearly the charges she was levying against Moscow.
"We have confirmed that a deadly toxin was found in Alexei Navalny's body and that toxin has been identified as a toxin that is found in Ecuadorian dart frogs," she said.
"Only the Russian government had the means, the motive and the opportunity to use that toxin against Alexei Navalny in prison and that is why we are here today to shine a spotlight on the Kremlin's barbaric attempt to silence Alexi Navalny's voice. To show that the Russian government has contempt for its citizens and the willingness to use this deadly toxin."
Yulia Navalnaya, Mr Navalny's widow, also spoke. She learnt about her husband's death while at the Munich conference on 16 February, 2024.
"It was the most horrible day in my life," she recalled, visibly upset.
"I came to the stage and I said my husband Alexei Navalny was poisoned. What could else happen with Putin's number one enemy in Russian prison?
"But - sorry, it is very difficult for me to say this - what could else happen with my husband in Russian prison? But now I understand and I know it is not just words. It is scientific proof."
British scientists played a key role in identifying the deadly neurotoxin - called epibatidine - that was allegedly administered to Mr Navalny.
The poison is found in the skin of the South American dart frogs, although Sky News understands it is likely the version used in the alleged murder was manufactured in a laboratory.
The effects of the poison are devastating, causing paralysis, respiratory arrest and an agonising death.
If - as claimed by Britain - the Kremlin did choose to use such an exotic substance to silence a critic, it demonstrates an unusual level of ruthlessness.
The Kremlin is already accused of trying to kill Mr Navalny in 2020 with a Novichok nerve agent - the same chemical weapon deployed against a former Russian spy on the streets of Salisbury two years earlier.
Fighting back, Moscow is well-practised and highly adept in the information space.
Its embassy in London was quick to respond to the allegations about dart frog poisonings.
"The goal of this ridiculous circus performance is transparent: to stoke waning anti-Russian sentiment in Western society. If there's no pretext, they laboriously invent one," it said.
"The method chosen by Western politicians - necropropaganda - is truly shocking. This isn't a quest for justice, but a mockery of the dead. Even after the death of a Russian citizen, London and European capitals cannot give him peace, which speaks volumes about the instigators of this campaign."
SpaceX delivered the US, French and Russian astronauts to the orbital research laboratory 277 miles (446km) up in space, a day after they launched from Cape Canaveral.
The new crew members include NASA's Jessica Meir and Jack Hathaway, France's Sophie Adenot and Russia's Andrey Fedyaev.
The last group of astronauts were forced to evacuate after one of them suffered what officials described as a serious health issue.
The withdrawal left only three crew members on board the space station - one American and two Russians - prompting Nasa to pause space walks and reduce research output.
Ms Meir, a marine biologist, and Mr Fedyaev, a former military pilot, have previously lived on the space station.
On her first mission to the station in 2019, Ms Meir took part in the first all-female spacewalk.
Ms Adenot, a military helicopter pilot, is only the second French woman to fly in space.
Mr Hathaway is a captain in the US Navy.
Video of the crew's arrival shared by NASA shows the four newcomers floating through the hatch from the SpaceX Dragon spacecraft into the space station.
"Bonjour," Ms Adenot said.
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The seven space explorers hugged and exchanged exuberant high-fives in the zero-gravity environment before posing for photos.
"Let's get rolling," Ms Meir said.
The crew will spend eight to nine months on board the International Space Station.
Last month's medical evacuation was NASA's first in 65 years of space exploration.
NASA has declined to divulge the identity of the astronaut who fell ill in orbit on 7 January or provide details of what happened, citing privacy concerns.
The ailing astronaut and three others returned to Earth more than a month sooner than planned.
They spent their first night back in a hospital before returning to Houston.
Demi Edmunds, 17, died at the scene of the crash on the A4042 in Cwmbran, Monmouthshire, at around 12.25pm on Thursday, Gwent Police said.
The teenager from nearby Caldicot, was the only pedestrian involved in the incident.
Her brother, Jake, said in a tribute published by Gwent Police that she was "the best sister I could've ever asked for. She was funny, kind and caring.
"Me and her were very close, she was like a best friend to me that I could trust with anything and everything, but she's no longer with us any more.
"Hopefully she's in heaven, somewhere better."
The girl's family, who police said have asked for privacy, said in a shared statement that Demi was "a beloved daughter, granddaughter, sister, cousin and niece.
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"Demi loved her friends, and she was loved by all", they said, describing her as "once met, never forgotten".
Detectives have appealed for the public's help, including anyone with CCTV or dashcam footage of the incident.
This apparently followed "higher-than-anticipated demand".
Providing condoms for athletes has been a gift from organisers - and a constant fascination to the world - for decades.
"We can confirm that condom supplies in the Olympic villages were temporarily depleted due to higher-than-anticipated demand," the Italian organising committee said in a statement on Saturday.
"Additional supplies are being delivered and will be distributed across all villages between today and Monday."
Some 300,000 condoms were provided for more than 10,500 athletes at the 2024 Paris Summer Games, but the stock was originally much lower for these games.
"I think 10,000 have been used, 2,800 athletes - you can go figure, as they say," International Olympic Committee spokesman Mark Adams said Saturday.
"It clearly shows that Valentine's Day is in full swing in the village."
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Condoms are traditionally popular for Olympic athletes to take home as "a kind of gift" to friends, Alpine skier Mialitiana Clerc said on Saturday.
"I'm not so shocked. I saw it in Beijing already," said Ms Clerc, who also competed for Madagascar four years ago at the Winter Games in China.
"There were some boxes with a lot of condoms at the entrance of every building where we were staying at the village."