Looking relaxed and smiling, she appeared on the steps of King Edward VII’s Hospital in central London and walked to her waiting limousine.
The Queen was admitted yesterday for assessment by doctors after being struck down by the stomach bug on Friday.
Her illness forced her to cancel a trip to Rome to meet the country’s President Giorgio Napolitano for a private lunch on Thursday.
Being admitted raised concerns about the Queen’s health generally as this was the first time the 86-year-old has been treated overnight at hospital in 10 years.
But she looked well and cheerful as she said goodbye to a uniformed member of staff who curtseyed to her.
The Queen, who wore a scarlet coat, was driven away by a chauffeur and is likely to spend time convalescing at her favourite home Windsor Castle.
A Buckingham Palace spokeswoman declined to discuss in detail her condition and just said that she remained in “good spirits”.