North Korea said he had fallen ill shortly after beginning a sentence of 15 years hard labour for attempting to steal a propaganda sign from a hotel.
He was given a sleeping pill after becoming ill after his trial last year and did not wake up, North Korea said.
Joseph Yun, the US state department's special envoy to North Korea, travelled to Pyongyang with a medical team to secure his release.
Otto Warmbier's family said they had not heard from him for more than a year and were only told last week that he had slipped into a coma shortly after his trial in March 2016.
Fred and Cindy Warmbier have said they were told he fell ill from botulism. They have said they believe he was "brutalised and terrorised by the pariah regime" in North Korea.
Who is Otto Warmbier?
- An economics student from the University of Virginia, originally from Cincinnati, Ohio.
- Was in North Korea as a tourist with Young Pioneer Tours when he was arrested on 2 January 2016.
- Convicted of trying to steal a propaganda plaque as a "trophy" for a US church
- In a tearful confession many think was forced, he said his aim "was to harm the motivation and work ethic of the Korean people".
North Korea's KCNA state news agency said Mr Warmbier was sent back home immediately after a court ruling on 13 June, but gave no further details.
US officials have asked for further information on how Mr Warmbier became so ill.
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