A companion said the 26-year-old was drunk and toppled from a level five floors up about 2.50am.
The tourist suffered head injuries, internal injuries and probably broken legs in the fall at the Sea Sand and Sun Resort and Spa in Kalim, north of Patong, on Phuket's west coast.
While his lower body appears to have landed on a concrete path, his torso and head may have been cushioned by a garden bed.
The pair arrived on Phuket yesterday and intended to stay until September 30.
The injured man was originally treated at Patong Hospital then transferred to the better-equipped Vachira Phuket Hospital in Phuket City, where he was being cared for in the intensive care unit.
The rooftop at the resort is surrounded by a low fence. Ground floors in Thailand count as the first floor.