Stephen McIlvenna, 39, of Northern Ireland, crashed on the Mountain Mile section of the TT course on Tuesday.
Race organizers said he was an experienced competitor who had been racing in the event for nine years.
Stephen McIlvenna won the 2009 Junior MGP and went on to compete in the TT.
A Manx Motor Cycle Club spokesman said it passed on its “deepest sympathy to Stephen’s family and friends”.
The coroner has been informed and an investigation into the circumstances of the accident is under way.
Northern Ireland’s Sports Minister Carál Ní Chuilín expressed her shock and sadness at Stephen McIlvenna’s death.
“Stephen was an experienced rider who clearly loved the sport of motorbike racing,” she said.
“He was a regular competitor on the Isle of Man and his death is a tragedy.”
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