Alexei Navalny, the late Russian opposition leader and a fierce critic of President Vladimir Putin, expected to die in prison, newly released excerpts of his memoir reveal.

“I will spend the rest of my life in prison and die here,” Navalny wrote in a March 22, 2022 diary entry, about two years before he died in an Arctic Circle jail of what correction officers claimed was “sudden death syndrome.”

“There will not be anybody to say goodbye to,” the married father of two continued. “All anniversaries will be celebrated without me. I’ll never see my grandchildren.”

Russian authorities claimed the dissident felt “unwell” and collapsed during a walk in February at the IK-3 penal colony in remote Kharp, about 1,200 miles northeast of Moscow — but his family and world leaders have blamed Putin.

Navalny, who was 47 at the time of his death, added that his approach was not one of “passivity.”

“I am trying to do everything I can from here to put an end to authoritarianism (or, more modestly, to contribute to ending it),” he wrote.

To cope, he said he would “imagine, as realistically as possible, the worst thing that could happen. And then (…) accept it.”

The New Yorker magazine published excerpts of the memoir, “Patriot,” Friday, ahead of of its scheduled Oct. 22 release.

Navalny started writing the book, which its publisher Alfred A. Knopf called his “final letter to the world,” while receiving treatment after being poisoned in 2020.

He blamed the attempt on his life on the Kremlin. Russian officials denied involvement in both his near-fatal poisoning and death.

He recuperated from the nerve agent in Germany and returned to Russia in 2021, where he was promptly taken into custody and sentenced to decades in prison on charges that included embezzlement and running an extremist group.

Navalny’s fellow inmates and prison guards would ask him why he voluntarily returned to Russia, according to his book.

“I don’t want to give up my country or betray it,” he explained in an excerpt from Jan. 17, a month before his death.

“If your convictions mean something, you must be prepared to stand up for them and make sacrifices if necessary,” he wrote.

Navalny gained prominence by organizing protests against Putin and forming teams across the country to expose corruption.

He displayed a sense of humor in the book, writing of a bet he made with his lawyers on what the length of an upcoming prison sentence would be.

“Olga reckoned eleven to fifteen years,” he wrote. “Vadim surprised everyone with his prediction of precisely twelve years and six months. I guessed seven to eight years and was the winner.”

Navalny’s widow, Yulia Navalnaya, said his story should “inspire others to stand up for what is right and to never lose sight of the values that truly matter.”

“Through its pages, readers will come to know the man I loved deeply — a man of profound integrity and unyielding courage,” Navalnaya said in a statement in April.

Half a million copies are set to be published in the US, with a simultaneous release in multiple countries. It has been translated into 11 different languages.

With Post wires