This article is more than
7 year oldThe US secretary of state said the 22-year-old was on his way home to the US.
The Warmbier family told US media they had found out only last week that their son has been in a coma since shortly after his trial in March 2016.
Mr Warmbier was sentenced to 15 years of hard labour for attempting to steal a propaganda sign from a hotel.
Who is Otto Warmbier?
Otto Warmbier is an economics graduate from the University of Virginia, originally from Cincinnati, Ohio.
He was in North Korea as a tourist with Young Pioneer Tours when he was arrested on 2 January 2016.
He gave a televised confession a month later, in which he said he had tried to take the sign as a "trophy" for a US church, adding "the aim of my task was to harm the motivation and work ethic of the Korean people".
It was not clear whether he had made the statement voluntarily, but foreign detainees in North Korea have previously recanted confessions, saying they were made under pressure.
After a short trial on 16 March, Mr Warmbier was given a 15-year prison sentence for crimes against the state.
His parents Fred and Cindy told CNN earlier this month that they had had no contact with their son for more than a year.
They spoke out about their fears for his safety as tensions rose after North Korea carried out missile tests and US warships were deployed to the region.
In a statement, Fred and Cindy said: "Otto has left North Korea. He is on a Medivac flight on his way home.
"Sadly, he is in a coma and we have been told he has been in that condition since March 2016. We learned of this only one week ago."
They were quoted by the Washington Post as saying they had been told Otto contracted botulism, a rare illness that causes paralysis, soon after his trial in March 2016.
He was given a sleeping pill, and has been in a coma ever since, the newspaper said.
Mr Warmbier's release comes hours after US basketball star Dennis Rodman arrived in North Korea, but it is not clear if the two events are linked.
Mr Rodman is a friend of North Korean leader Kim Jong-un and has made several visits to the country in recent years.
He said upon his arrival this time that he was in the country as a private citizen, and "my purpose is to actually see if I can keep bringing sports to North Korea".
Three other US citizens remain in custody in North Korea. They are 62-year-old Kim Dong-chul, a Korean-American missionary; Korean-American professor Kim Sang-duk (or Tony Kim); and Kim Hak-song, who worked at the Pyongyang University of Science and Technology (PUST).
US Secretary of State Rex Tillerson, in his statement, said the US state department "continues to have discussions" with North Korea over the remaining three detainees.
He said they would make no further comment on Mr Warmbier "out of respect for the privacy" of his family.
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