The 19-year-old man had to be cut from the wreckage before being flown by air ambulance to Raigmore Hospital in Inverness.
It followed a two-vehicle collision on the A82 Inverness to Fort William road two miles north of Invermoriston at about 8.25am.
The vehicles involved were a Vauxhall Corsa being driven north by a 19-year-old man and a VW Golf, with a 46-year-old man and a 44-year-old woman on board, travelling south.
The Corsa left the road and went down a steep embankment halfway towards the loch. The occupants of the other car suffered minor injuries and were taken by ambulance to Raigmore Hospital.
The road was closed for four-and-a-half hours due to the difficulty in rescuing the casualty from the steep embankment and to allow a full collision investigation.