The jailed dissident was aged 47.
The jailed dissident was aged 47.
Russian President Vladimir Putin has been informed of his death, according to the state news agency TASS.
According to Sky News, Russia's investigative committee said it had launched a procedural investigation.
The Federal Penitentiary Service for Yamal said in a statement: "On 16 February, 2024, in penal colony No 3, convict AA Navalny felt unwell after a walk, almost immediately losing consciousness.
"The facility's medical workers immediately arrived at the scene and an emergency medical team was called in.
"All necessary resuscitation measures have been carried out, but they did not yield positive results.
"Emergency medics confirmed the death of the convict.
"The causes of death have been established."
Nobel Peace Prize winner and campaigning journalist Dmitry Muratov, who is editor-in-chief of Russia's most famous independent newspaper, Novaya Gazeta, described the death of Mr Navalny as "murder" and said he believed prison conditions had led to his demise.
French Foreign Minister Stephane Sejourne said Mr Navalny has paid with his life for his "resistance to a system of oppression".
Mr Sejourne added: "His death in a penal colony reminds us of the reality of Vladimir Putin's regime."
Sky News' Moscow correspondent Diana Magnay said: "It's appalling. It's an absolute tragedy and it's a stain on the conscience of the Russian state.
"His health has been deteriorating... he has been held in solitary confinement for much of the time since he was imprisoned after his return to Russia in January 2020.
"But it does seem very shocking that his health should have deteriorated to the extent that he's dead now - given we have seen him in court videos, we have also had a lot of social media presence from him via his lawyers where he is constantly seen upbeat and cheerful.
"Yes, he's been put through a lot by the prison systems but we have not had an indication that he would die this suddenly, so I just wonder if something specific happened to him."
She added: "It will come as a huge shock to his supporters.
"He was only recently moved to this prison colony up in the very far north of the country, and it is an absolute tragedy for any notion of a free Russia, a Russian democracy, which was all that Alexei Navalny devoted his life to since he burst onto the scene in 2012.
"He has been a moral voice for Russia from his prison cell, against the war and against the lack of freedoms that he felt for his fellow countrymen. That's why he returned to Russia after he was poisoned.
"I think a lot of people hoped that one day he would leave prison and that he might be a leader for the Russian people - but the fact that he is now dead, only a few weeks before Russia holds an election... I don't think will play very well for Vladimir Putin.
"To see the one man who did stand as a viable opposition politician - even if he was jailed - to see that man dead."