Kremlin critic Arkady Babchenko, who was reported to have been shot dead on Wednesday, has appeared at a news conference.
Vasily Gritsak, head of the Ukrainian Security Service, told reporters the agency faked Mr Babchenko’s death to stop a Russian plan to kill him.
Russia said it was happy that Mr Bachenko was alive after all.
Kiev had said the 41-year-old, a strong critic of Vladimir Putin, was shot multiple times in the back at his apartment building while on his way to buy bread.
They had claimed he was found bleeding by his wife.
But, remarkably, Mr Babchenko showed up at the news conference, apologised for what people had been through, and thanked everyone who had been mourning his death.
He thanked the Ukrainian security service for saving his life and said the operation had been planned for two months.
Mr Babchenko, one of Russia’s best-known war reporters, spoke and wrote last year about leaving the country because of repeated threats that he and his family would be harmed.
Ukraine had released an image of a man wanted in connection with the killing.
A Ukrainian citizen has been arrested.
Mr Gritsak said his service had received information about a Russian plan to assassinate Mr Babchenko, and had managed to prevent it.
He added that a Ukrainian citizen had been recruited and paid $40,000 (£30,130). That citizen was also apparently asked to buy weapons and ammunition.
Russia said Ukraine’s alleged use of Mr Babchenko served its propoganda purposes.